21st April 2026

If your business is still paying for ISDN lines or traditional PSTN connections, you’re almost certainly overpaying. SIP trunking has been saving UK businesses 40-60% on their telecoms costs — and with the PSTN switch-off underway, it’s no longer optional to understand it.

What Is SIP Trunking?

SIP trunking replaces traditional physical phone lines with virtual lines that run over your internet connection. Instead of paying for fixed ISDN channels, you have a pool of virtual channels that handle multiple simultaneous calls — all transmitted as data over your broadband or leased line.

Why Is It Cheaper?

  • No line rental per channel — SIP uses concurrent call pricing, so you only pay for capacity you actually need
  • Lower call rates — SIP providers compete aggressively, particularly for international calls
  • Geographic number flexibility — keep existing numbers or add any UK area code without physical presence there
  • No engineer visits — adding or removing lines is done in software
  • Works with your existing IP PBX — no need to replace your phone system infrastructure

The PSTN Switch-Off

Openreach is retiring the PSTN and ISDN by January 2027. Traditional analogue and ISDN lines will simply stop working. Every UK business needs to migrate to IP-based telephony — and SIP trunking is the primary replacement for businesses with existing PBX systems. If you’re still on ISDN, the clock is ticking.

What to Look for in a UK SIP Provider

  • UK-based infrastructure for lower latency
  • Sufficient concurrent call capacity for your busiest periods
  • Failover options if your internet connection goes down
  • Number porting for your existing numbers
  • A clear SLA and responsive support

A Common Gotcha

SIP ALG on many routers interferes with SIP traffic. Most SIP providers will advise you to disable it during setup — it’s one of the most frequent causes of call quality problems.

At Just Business Phones, we manage SIP migrations end-to-end — including number porting, PBX configuration, and network readiness checks.

Ready to upgrade your business phone system? Get in touch with Just Business Phones today for a free consultation — we’ll help you find the right solution at the right price.

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