SIP Trunking, 3CX & Hybrid Phone Systems UK

SIP trunking, 3CX, on-premise IP PBX and hybrid phone systems help UK businesses modernise calling without replacing every existing system at once. They suit companies with PBX hardware, number ranges, multi-site estates, technical control needs or phased migration plans before PSTN and ISDN services are fully retired safely by 2027.

Last updated: June 2026
Reviewed by: CompareServices Editorial Team
Reading time: 17–20 minutes

SIP trunking 3CX on-premise PBX and hybrid phone systems for UK businesses — SIP trunking, 3CX, on-premise PBX and hybrid phone-system routes for UK businesses planning phased migration.

Quick Answer

SIP trunking, 3CX, on-premise IP PBX and hybrid phone systems are best suited to UK businesses that already have phone infrastructure, technical support, number ranges, multi-site routing or migration constraints. They are not usually the simplest option for a very small team that only needs basic app-based calling.

The key decision is whether you want to keep and modernise existing PBX infrastructure, move to a software PBX such as 3CX, maintain local PBX control, or migrate in phases using a hybrid model.

Business situation	Best-fit route	Why it fits	Main risk to check
Existing PBX still works	SIP trunking	Replaces legacy line connectivity while keeping compatible PBX features	PBX compatibility, security and failover
IT-led business wants software PBX control	3CX phone system	Supports PBX-style control, SIP trunks, apps and flexible deployment options	Hosting, licensing, updates and support ownership
Regulated or complex environment	On-premise IP PBX	Offers more local control over configuration, access and data handling	Maintenance, resilience and internal expertise
Multiple sites with different readiness levels	Hybrid phone system	Lets the business move users, numbers or sites in stages	Long-term complexity and double-running costs
Small team with no PBX investment	Hosted cloud phone system	Simpler to manage and usually faster to deploy	May not give the same technical control

Best rule: if you do not already have a PBX, SIP trunking or internal IT ownership, compare cloud phone systems first. If you do have existing PBX infrastructure, this guide helps you decide whether to keep, upgrade, replace or phase it out.

Why This Guide Exists

Most business phone system guides focus on fully hosted cloud VoIP platforms. That is useful for many SMEs, but it does not answer every real-world migration scenario.

Some UK businesses already have:

A working PBX with extensions, reception routing and number ranges
Existing desk phones, call queues and branch-level call flows
Internal IT or a telecoms partner that manages telephony
Regulatory or operational reasons to keep tighter control
Multiple sites that cannot be migrated at the same time
Legacy ISDN or PSTN connectivity that needs replacing before final retirement

For those organisations, the question is not simply “which cloud phone provider is cheapest?” The question is whether to use SIP trunks, 3CX, an on-premise IP PBX, a hybrid route or a full cloud replacement.

What Is SIP Trunking?

SIP trunking is a way of connecting a business phone system to external phone networks using internet protocol. Instead of using older ISDN-style circuits, the business uses SIP trunks delivered over a suitable data connection.

In practice, SIP trunking is usually considered when a business wants to keep a compatible PBX but replace legacy line connectivity. It can support number ranges, direct dial numbers, call routing and multiple concurrent calls.

What SIP trunking normally includes

SIP channels for concurrent calls
Number presentation and number ranges
Support for inbound and outbound calling
Connection to an existing compatible PBX
Disaster recovery or failover routing where configured
Lower reliance on legacy PSTN or ISDN circuits

When SIP trunking is a good fit

Your PBX is still suitable and supported
You have invested in handsets, call routing or internal telephony skills
You need to keep branch or department routing stable during migration
You want to replace ISDN without changing every user workflow at once
You understand that SIP still needs broadband, security and support planning

When SIP trunking may not be the best route

You have no existing PBX investment
Your PBX is old, unsupported or difficult to secure
You want app-based calling with minimum management responsibility
You need voice, video, messaging and collaboration in one hosted platform
You do not have internal or partner support for PBX administration

What Is A 3CX Phone System?

3CX is a software-based business phone system that can be used as a PBX-style platform with SIP trunks, extensions, desktop and mobile apps, call routing and administration controls. It is often considered by businesses that want more flexibility than a simple hosted VoIP subscription while avoiding some traditional hardware PBX limitations.

3CX should not be treated as “just another cloud phone provider”. The buying model, deployment responsibility and support model can be different. Depending on the setup, a business may need to consider software licensing, hosting, SIP trunk provider costs, handsets, security, updates and partner support.

3CX consideration	What to check before choosing
Deployment route	Hosted, partner-managed, self-managed or on-premise options
SIP trunk provider	Supported provider, call bundles, channels, number porting and failover
Licensing model	Edition, simultaneous call limits, features and renewal terms
Support owner	Whether support is from 3CX, a partner, internal IT or all three
Security responsibility	Updates, firewall rules, access controls, backups and monitoring
Feature fit	Call queues, reporting, CRM integration, recording, apps and admin controls

Important: do not compare 3CX only against per-user cloud VoIP prices. Compare the full first-year cost including software, hosting, SIP trunks, support, handsets, migration and internal time.

What Is An On-Premise IP PBX?

An on-premise IP PBX is a business phone system hosted and managed on infrastructure controlled by the organisation or its telecoms partner. It connects users, extensions and call routing internally, then connects to external calling using SIP trunks or another digital route.

This model is less common for smaller companies now, but it still has a place where technical control, custom call routing, security policy, data handling or operational continuity require more than a standard hosted cloud phone system.

Typical reasons to keep or deploy on-premise IP PBX

Strong internal IT ownership
Complex site-to-site call routing
Regulated-sector governance requirements
Existing investment in PBX, phones and support contracts
Specialist integrations that are difficult to replicate in cloud
Preference for local control over updates, access and configuration

Trade-offs to understand

Higher internal responsibility
Security and updates must be actively managed
Backup, resilience and disaster recovery need clear design
Scaling may need more licensing or infrastructure work
Support depends on in-house skills or a specialist telecoms partner

What Is A Hybrid Business Phone System?

A hybrid phone system mixes more than one telephony model. It may combine a legacy or IP PBX with SIP trunks, cloud users, hosted call routing, remote apps or phased site migration.

Hybrid is not automatically better than cloud. It is useful when the business cannot safely move everything in one go. The danger is allowing the hybrid setup to become permanent without a clear end-state plan.

Hybrid use case	Why businesses choose it	Risk to manage
Multi-site estate	Move sites in phases rather than forcing a single cutover	Inconsistent user experience and support complexity
PBX still under contract	Avoid wasting existing investment immediately	Double-running costs if cloud and PBX both remain active
Legacy connected devices	Test alarms, lifts, fax or payment devices separately	Assuming every PSTN dependency has been migrated
Department-by-department migration	Move lower-risk teams before critical call groups	Confusing call routing if the plan is not documented
Technical control requirement	Keep core routing controlled while cloud users are added	Security and fault ownership gaps

SIP Trunking vs 3CX vs On-Premise PBX vs Hybrid

Route	Best for	Commercial model	Control level	Complexity	Remote working fit
SIP trunking	Existing PBX users replacing ISDN	Channels, numbers, calls and support	Medium to high	Medium	Depends on PBX
3CX	IT-led firms wanting software PBX control	Software, hosting, SIP and support	Medium to high	Medium	Good if configured well
On-premise IP PBX	Regulated or complex organisations	Hardware/software, support and SIP	High	High	Moderate to good with design
Hybrid phone system	Phased migrations and mixed estates	Mixed cloud, SIP, PBX and support	Mixed	High	Varies by site/team
Hosted cloud phone system	Most SMEs without PBX constraints	Per user/month	Lower	Lower	Strong

Costs And Quote Factors

Infrastructure-led phone systems are harder to compare than simple per-user hosted VoIP because the price can be shaped by channels, licences, hosting, support, hardware, call traffic and migration work.

Cost items to separate in every quote

SIP trunk or channel charges
Number rental and number ranges
Inbound and outbound call bundles
Software licences or PBX edition
Hosting, server or appliance costs
Support and maintenance contract
Implementation and call-flow configuration
Security hardening, firewall and SBC work
Desk phones, headsets and adapters
Backup and disaster recovery route
Number porting and temporary forwarding
Training and handover documentation

Quote comparison table

Cost area	SIP trunking	3CX	On-premise PBX	Hybrid
Upfront setup	Medium	Medium	High	Medium to high
Monthly user cost	Not always per-user	Depends on licence/support model	Lower user licence, higher support burden	Mixed
Technical ownership	Shared with PBX/support partner	Internal/partner-led	Mostly internal/partner-led	Shared and more complex
Best value when	PBX still has useful life	IT can manage or partner can support it	Control requirements justify overhead	Phased migration reduces disruption

Migration Checklist For Existing PBX, SIP And 3CX Routes

Use this checklist before approving a SIP, 3CX, PBX or hybrid migration plan.

Audit all active numbers. Include main numbers, DDIs, hidden numbers, branch numbers and lines used for devices.
Confirm PBX status. Check make, model, software version, support status, warranty and compatibility with SIP.
Map current call flows. Document reception, departments, hunt groups, queues, out-of-hours routing and failover.
Check connected devices. Include fax, alarms, lift phones, door entry systems, payment terminals and fire panels.
Test the data connection. SIP and software PBX routes still need suitable broadband or dedicated connectivity.
Define emergency calling behaviour. Confirm 999 access, location data and what happens during power or internet outages.
Plan number porting. Do not cancel old services before numbers are confirmed live on the new route.
Design failover. Decide where calls route if the PBX, SIP trunk, broadband or site fails.
Confirm support ownership. Avoid gaps between broadband provider, SIP provider, PBX partner and internal IT.
Run a pilot. Test a small group, branch or non-critical number before full cutover.

Security And Resilience Checks

SIP, 3CX, self-managed PBX and hybrid systems need stronger governance than a simple provider-managed phone service. The flexibility is useful, but it can introduce security and fault-management risk if ownership is unclear.

Do not skip this stage: exposed PBX systems, weak passwords, missing updates and poorly configured SIP access can create fraud and outage risk.

Minimum security checks

Use strong admin credentials and role-based access
Restrict management access to trusted networks or secure VPN where appropriate
Apply PBX and 3CX updates promptly
Use supported SIP providers and secure trunk configuration
Disable unused extensions, routes and international destinations
Monitor unusual call patterns and failed login attempts
Document backup and recovery processes

Minimum resilience checks

Secondary call route or failover number
Backup broadband or mobile failover for critical sites
Power resilience for routers, switches and PBX hardware
Tested recovery process for PBX or hosting failure
Clear escalation route for faults across all suppliers

Questions To Ask SIP, 3CX And PBX Providers

Is our current PBX compatible with your SIP trunks?
How many SIP channels do we actually need?
Can we keep all existing numbers and DDI ranges?
What happens if our internet connection fails?
Who owns security updates and PBX maintenance?
How is 999/emergency calling handled?
Are international and premium-rate calls restricted by default?
Can we block fraud-prone destinations?
What monitoring and call-spend alerts are available?
How will number porting be staged?
What does the first-year cost include?
What support is available during and after migration?
Can the setup later move fully cloud if required?

Frequently Asked Questions

What is SIP trunking?

SIP trunking connects a compatible business phone system or PBX to external phone networks over internet protocol. It replaces traditional ISDN-style connectivity while allowing the business to keep suitable PBX hardware, extensions, number ranges and call routing under its own control.

Is SIP trunking the same as VoIP?

SIP trunking uses VoIP technology, but it is not the same as a fully hosted cloud phone system. SIP trunks usually connect an existing PBX to digital calling, while hosted cloud VoIP replaces most or all of the PBX infrastructure with a provider-managed platform.

What is a 3CX phone system?

3CX is a software-based business phone system that can support SIP trunks, extensions, softphones, call routing and PBX-style control. It is often considered by IT-led businesses that want more control than a simple fully hosted cloud phone system.

Is 3CX better than a hosted cloud phone system?

3CX can be better where technical control, SIP flexibility, deployment choice and PBX-style administration matter. A hosted cloud phone system is often better for businesses that want simpler provider-managed calling, faster setup and less internal technical responsibility.

Who should consider an on-premise IP PBX?

An on-premise IP PBX can suit regulated organisations, IT-managed firms, sites with complex routing or businesses that want strong local control. It is less suitable for companies that do not want to manage updates, resilience, security, backups and specialist support.

What is a hybrid business phone system?

A hybrid business phone system combines existing PBX infrastructure, SIP trunks, cloud services or hosted users during a phased migration. It is useful where a business cannot replace every site, device or call flow at the same time.

Can SIP trunking replace ISDN?

Yes, SIP trunking is a common route for replacing ISDN where the existing PBX is compatible or can be upgraded. Businesses should check PBX support, number ranges, channels, emergency calling, failover and security before ordering SIP trunks.

How many SIP channels does a business need?

SIP channel requirements depend on concurrent calls, not only user count. A 40-user business might need far fewer than 40 channels if only 10 to 15 calls happen at once, but busy sales, support or reception teams may need more capacity.

Can I keep my existing phone numbers with SIP trunking or 3CX?

Usually yes, but number porting must be planned carefully. Check number ownership, current contract terms, number ranges, DDI allocation, porting lead times and fallback routing before cancelling the old service.

Is a hybrid phone system more expensive than moving fully cloud?

It can be. Hybrid systems may protect existing investment and reduce migration risk, but they can also create two environments to manage. Compare short-term migration savings against support, licensing, SIP trunk, hardware and long-term maintenance costs.

What are the biggest risks with SIP, 3CX or self-managed PBX systems?

The main risks are weak security, poor failover, unmanaged updates, unclear support ownership, bad call routing design, number-porting disruption and underestimating internal technical responsibility. These systems need proper governance and documented support processes.

Should small businesses use SIP trunking or 3CX?

Some technically confident small businesses can use SIP trunking or 3CX successfully, but many small businesses are better served by a simple hosted cloud phone system. The right choice depends on support skills, existing hardware, call complexity and control requirements.

Compare SIP, 3CX And Hybrid Phone System Routes

CompareServices helps UK businesses understand whether SIP trunking, 3CX, on-premise IP PBX, hybrid migration or a fully hosted cloud phone system is the best fit.

What you can compare:

SIP trunk and channel requirements
3CX and software PBX suitability
Existing PBX compatibility
Number porting and DDI planning
Hybrid migration risk
Support and maintenance ownership
Failover, security and emergency calling

Start comparing infrastructure-led phone-system routes now.

SIP Trunking, 3CX, On-Premise PBX & Hybrid Phone Systems UK (2026)

Keep, upgrade or replace?

Choose The Right Route Before Replacing A Working PBX

SIP Trunking

3CX Phone System

On-Premise IP PBX

Hybrid Migration

Quick Answer

Why This Guide Exists