If your business still has an on-site phone system connected by ISDN lines, SIP trunks are usually the simplest way to move to the new all-IP network without replacing the phone system itself. Here's what they are, what to think about, and how to get set up.
What Are SIP Trunks?
A SIP trunk connects your existing phone system (PBX) to the phone network over an internet connection, instead of a physical ISDN line. Your staff keep using the same handsets and business numbers; only the way calls reach the phone network changes.
Why Businesses Are Switching
- ISDN is being withdrawn — the UK's ISDN and PSTN switch-off is confirmed for 31 January 2027, so lines that still rely on it need a migration plan.
- Usually lower running costs — IP connectivity generally means lower monthly rental than ISDN, and often lower call costs too.
- Keep your numbers when you move — with a SIP connection you can move office and keep the same geographic number, without paying for ongoing call forwarding.
- Better business continuity — if your office has to be relocated in an emergency, SIP trunks can be re-pointed far more easily than physical lines.
- Fewer systems at multi-site businesses — SIP trunking can help rationalise ISDN lines and reduce the number of PBXs you need to maintain.
What to Think About Before Choosing
- How many calls you need at once — SIP trunks are sized by channels, so work out your busiest-hour call volume rather than just your headcount.
- Whether your phone system supports SIP — many on-site systems do, some older ones need an adapter or an upgrade, and a partner can check before you commit.
- The quality of your internet connection — voice depends on a stable connection, so a backup is worth considering. See our business internet failover page.
- Keeping your existing numbers — almost always possible, but confirm it with any provider before you switch.
- SIP trunks or a hosted phone system? — if your on-site hardware is ageing, moving the whole system to the cloud may make more sense. Our hosted PBX vs SIP trunks comparison covers the trade-offs.
Get connected with an accredited Plustel partner to discuss SIP trunks for your business.
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How Plustel Fits In
Plustel's SIP connectivity is designed as a direct replacement for ISDN circuits, with a 24/7 portal where new SIP trunks can be added and managed. As with all our services, we work entirely through accredited partners rather than selling directly, so getting set up means being connected with a partner who can assess your phone system and arrange the move.
If you'd like to understand the technology first, our SIP trunking guide covers it in more depth. Otherwise, get in touch below and we'll connect you with a partner.