If your business still relies on ISDN or traditional analogue lines, you are on borrowed time. BT Openreach's retirement of the legacy public telephone network is scheduled for January 2027, which means choosing among SIP trunk providers is now a practical deadline, not a future project.

The good news is that moving to SIP trunking is usually straightforward, often cheaper, and gives you far more flexibility. The difficulty lies in separating providers who build reliable services from those who simply resell capacity. This article shows what to look for.

What a SIP Trunk Actually Does

A SIP trunk is a virtual phone line delivered over your internet connection. It connects your existing or new phone system to the public telephone network without physical ISDN circuits.

Instead of buying lines in fixed bundles of two or thirty, you buy voice channels. Each channel supports one simultaneous call. A ten-person office may need only six or eight, while a busy contact team might need forty.

SIP trunking works with an on-site PBX, and also with a cloud PBX or hosted telephony platform. That flexibility is one reason business SIP trunking has become the default replacement for ISDN.

Why UK Businesses Are Moving Now

The switch-off is the headline reason, but it isn't the only one. Businesses we see moving typically cite three things:

• Cost control. Line rental disappears, and calls to UK landlines and mobiles are usually bundled or priced per minute at modest rates.
• Flexibility. Adding a channel for a seasonal rush takes minutes rather than an engineer visit.
• Continuity. Calls can be rerouted to mobiles or another site if your office loses power or broadband.

Consider a Leeds-based accountancy firm with two offices and an old ISDN30 line at each. Moving both to one SIP trunk Provider setup lets them share channels, keep their existing numbers and cut duplicated rental charges.

What to Check Before You Choose

Network quality and call routing

Voice is sensitive to delay and packet loss. Ask where the provider's switches sit, whether they own their network or buy wholesale, and how many carriers they route through.

A provider with diverse routing across UK data centres is far less exposed to a single point of failure.

Number porting and local presence

You should be able to keep your existing 01, 02 or 03 numbers. Ask how porting works, how long it takes, and whether the provider schedules the cutover outside working hours. Poorly handled porting is where most migrations go wrong.

Resilience and failover

A good SIP provider offers automatic failover. If your primary connection drops, calls divert to another number, a mobile app or a second site. Ask whether this is included or charged separately.

Compatibility with your phone system

Check that the provider supports your PBX model and firmware. Some older systems need a gateway device, and a reputable provider will say so upfront rather than leaving you to discover it on installation day.

Security and fraud protection

Toll fraud, where criminals hijack a system and run up premium-rate calls, is a genuine risk. Look for spending caps, international call barring by default, and real-time monitoring.

Understanding Pricing Without the Surprises

Searching for the cheapest UK VoIP providers will turn up plenty of low headline rates, but the monthly cost depends on more than a per-channel price. Compare the following:

• Cost per channel and any minimum channel count
• Inclusive minutes versus pay-as-you-go rates
• Charges for calls to mobiles, 03 numbers and international destinations
• Number rental and porting fees
• Setup, contract length and early exit terms

Cheap is not automatically good value. A slightly higher channel price from a provider with proper support and resilient routing often costs less overall than a bargain service that drops calls during busy periods.

Choosing Between SIP Trunking and a Hosted Phone System

SIP trunking suits businesses that already own a decent PBX or need to keep specific hardware, such as a call recording system or a legacy integration.

If your phone system is old or you are opening new sites, a hosted solution may be the better route. Cloud telephony removes on-site hardware, bundles features such as call queues, auto attendants and mobile apps, and fits into wider unified communications tools.

Some UK providers offer both, which lets you start with trunks and migrate users to the cloud later. Wavetel Business is one example of a UK provider that supports this kind of phased approach, which suits companies that cannot move everything at once.

How a Sensible Migration Works

A well-run switch normally follows these stages:

1. Audit current lines, numbers and call volumes.
2. Test your internet connection for bandwidth and stability, ideally with a dedicated or prioritised voice path.
3. Agree the number of channels, with some headroom for peaks.
4. Port numbers on a scheduled date, with a fallback plan.
5. Test inbound, outbound and failover scenarios before going live.

Allow a few weeks rather than days, particularly if number porting is involved and the switch-off date is close.

Questions Businesses Commonly Ask

How many SIP channels does my business need?

Count the maximum number of simultaneous calls at your busiest time, then add a small buffer. Your current call logs usually give a reliable answer.

Do I need a new phone system?

Not always. Many modern IP PBXs work directly with a SIP trunk. Older digital systems may need a gateway or replacement.

What internet connection is needed?

A stable business-grade connection is strongly recommended. As a rough guide, each concurrent call uses a small amount of bandwidth, but quality of service matters more than raw speed.

Can I keep my existing phone numbers?

In most cases, yes. Numbers can be ported from your current provider under UK number portability rules.

Is SIP trunking the same as VoIP?

SIP trunking is one form of VoIP. VoIP is the broader term for voice delivered over IP networks, while a SIP trunk specifically connects a phone system to the public network.

The Takeaway

With the legacy network closing, the right choice of provider matters more than the price per channel. Focus on network quality, number porting, failover, security and honest pricing. Ask for references from similar-sized UK businesses, request a test period if possible, and plan the migration well ahead of the deadline.

A provider that explains the trade-offs clearly before you sign is usually the one that will support you properly afterwards.