What Is SIP Trunking? SIP Trunks vs Hosted VoIP for UK Businesses

Objective

This guide explains what a SIP trunk is, how it differs from hosted VoIP and which suits your business better. It’s written for owners, office managers and IT leads who need to replace ISDN lines without a headache.

Key Takeaways

• SIP trunking connects an existing compatible phone system to the telephone network using an internet connection.
• Hosted VoIP provides phone system functionality through a provider-hosted platform, reducing the need to maintain an on-site PBX.
• The main difference in the SIP trunk vs VoIP comparison is that SIP trunking connects a phone system, while hosted VoIP typically supplies the phone system too.
• SIP trunking is a great choice for businesses that want to keep their compatible PBX systems.
• Organisations looking for better management, remote employment, and fewer on-site systems might find hosted VoIP the right choice.
• Call quality depends on network performance, configuration, capacity, and provider service, not only connection speed.
• Before choosing SIP trunk providers in the UK, check compatibility, number porting, emergency calling, support and resilience.

Table of Contents

1. What Is SIP Trunking?
2. How Does a SIP Trunk Work?
3. Hosted VoIP vs SIP Trunking: What’s the Difference?
4. Which Is Right for Your Business: SIP Trunking or Hosted VoIP?
5. What to Check Before Choosing SIP Trunk Providers in the UK
6. Frequently Asked Questions
7. Conclusion 

SIP trunking lets a compatible business phone system make and receive calls through a provider’s telephone network using an IP connection. It is one option for businesses reviewing their phone systems ahead of the UK’s traditional landline switch-off, but it is not the right replacement for every setup.

Openreach’s deadline for retiring its traditional PSTN network is 31 January 2027. Businesses using legacy PSTN or ISDN services should speak to their provider about a suitable replacement and check any equipment connected to those lines. SIP trunking may allow a compatible existing phone system to continue operating, but other businesses may be better suited to hosted VoIP or another digital solution. Openreach’s business guidance explains what to check before switching.

What Is SIP Trunking?

SIP trunking connects your existing compatible business phone system to a telephone provider over an IP network, allowing you to make and receive calls without relying on traditional ISDN or analogue phone lines.

SIP stands for Session Initiation Protocol. It helps establish, manage and end voice calls between compatible systems. A SIP trunk acts as the connection between your private branch exchange (PBX) and the provider’s voice network 

For a business with a compatible PBX, SIP trunking can modernise telephone connectivity without requiring an immediate replacement of the entire phone system. Check compatibility, licensing and ongoing support requirements before committing.

For example, imagine a company with 40 employees using desk phones connected to an on-site PBX. Rather than replacing every handset and changing the whole system immediately, the business may be able to use SIP trunks to connect its compatible PBX to the telephone network.

The important word is compatible. Not every legacy phone system supports SIP trunking, so check the equipment, software and licensing before making a decision.

How Does a SIP Trunk Work?

Think of your PBX as the heart of business calling. It manages extensions, call routing, voicemail and other phone-system features. The SIP trunk provides the connection that allows those calls to reach external telephone numbers.

A typical setup works like this:

1. An employee dials a customer using their desk phone.
2. The PBX processes the call and sends it through the SIP trunk.
3. The provider sends the call to the destination telephone number.
4. Incoming calls travel back through the provider and are directed to the correct extension or department.

How many simultaneous calls can your firm make? This depends on the size of the trunk and your service contract. A busy sales team will probably need more concurrent calls than a modest workplace where telephones ring only now and then.

SIP trunking can also support business number management and call-routing arrangements, depending on the provider and configuration. However, the PBX still needs to be maintained, secured and supported. 

Hosted VoIP vs SIP Trunking: What’s the Difference?

Both use the internet to carry calls, which is why people mix them up. The real difference is where your phone system lives.

With SIP trunking, a compatible PBX manages your phone system while the SIP service connects it to the provider’s telephone network. With hosted VoIP, the provider hosts the phone platform, which staff can access through compatible desk phones or apps. The exact setup depends on the service.

Think of it this way: SIP trunking changes the road, while hosted VoIP changes the vehicle. Neither is automatically better. They solve different problems.

For more context, see ARC Communications’ guide to the best cloud phone systems for UK small businesses.

Which Is Right for Your Business: SIP Trunking or Hosted VoIP?

The decision becomes easier when you start with your current setup rather than comparing feature lists 

When SIP trunking makes sense

• You have a working PBX you’re happy with and don’t want to replace yet.
• Your staff mostly work in one office.
• You want a quick, low-disruption replacement for ISDN lines.
• You need to keep the same phone and call routing.

When hosted VoIP makes sense

• Your PBX is ageing and costly to maintain.
• Your team works from home, on the road or across several sites.
• You want to add or remove users without calling an engineer.
• You want to handle calls, voicemail and video from one place.

Still torn between the two? ARC’s advisers can look at your current phone system, your team and your timeline, then recommend what fits. Call 0115 950 8100 for a straight answer with no jargon.

What to Check Before Choosing SIP Trunk Providers in the UK

Comparing SIP trunk providers UK businesses can use involves more than looking at the monthly price or number of included calls. The service must work with your phone system and support the way your organisation handles customers.

Before signing, ask these questions:

• Is my PBX compatible? Confirm supported equipment, software versions, licences and any required session border controller.
• How many concurrent calls are included? Estimate peak demand, not just the number of employees.
• Can I keep my existing numbers? Confirm number porting arrangements, timelines and any restrictions before scheduling a migration.
• What happens during an outage? Ask about call diversion, alternative routing, backup connectivity and service restoration arrangements.
• What security is there? Talk about restrictions on access, fraud prevention, firewall setup, and monitoring.
• Who provides support? Establish who handles faults involving the trunk, internet connection and PBX, particularly if different suppliers manage each component.

A successful migration also needs testing. Check inbound and outbound calls, caller ID, voicemail, call queues and emergency calling arrangements before moving users across. Because internet-based phone services may stop working during a power cut or broadband failure without suitable backup, review your continuity arrangements too. Ofcom’s guidance on emergency access during power cuts explains the issue.

With the UK’s traditional telephone network transition approaching its January 2027 deadline for the Openreach PSTN network, businesses still relying on legacy services should review their options rather than leave the change until the last minute. The right solution depends on your current equipment and available services.

Frequently Asked Questions

Q1. What is a SIP trunk?

A SIP trunk is a service that provides IP communication between a compliant business phone system and a provider’s telephone network. Allows you to make and receive calls without traditional telephone connections.

Q2. Is SIP trunking the same as VoIP?

No. VoIP describes the technology used to carry voice over IP networks. SIP trunking is one way to connect a business phone system to external telephone networks using that technology.

Q3. Do I need a PBX for SIP trunking?

Usually, yes. You need a compatible PBX or another supported call-control platform to manage calls through the trunk. Hosted VoIP generally includes the central phone-system platform.

Q4. Can I keep my existing business phone numbers?

When you move services, you can generally transfer or change numbers. Before you move, confirm eligibility, provider requirements and the migration schedule.

Conclusion: Find the Right Business Phone Solution

SIP trunking lets you modernise how your firm makes and receives calls, while keeping your existing compatible phone system. If you prefer a provider-managed solution with less on-site upkeep, Hosted VoIP could be a better option.

ARC Communications helps UK businesses compare phone systems and connectivity options against their requirements, budget and existing setup.

Not sure which route makes sense for your business? Visit our ARC Communications’ business telephone systems page to explore your options and discuss a suitable solution