If TM has told you your PRI line is being retired, the replacement they offer is almost always MLS. Migrating from TM PRI to MLS swaps a dedicated E1/PRI voice circuit for Multi Line SIP — the same phone numbers and channels, now delivered as SIP over an internet connection. This guide explains what actually changes, how the cut-over runs, and why the smart move is to treat the switch as your chance to modernise the whole phone system.
PRI vs MLS: what's really changing
PRI (Primary Rate Interface, an E1 circuit) carries up to 30 concurrent voice channels over a dedicated copper/fibre line into your premises. MLS carries those same channels as SIP over a data connection — it rides TM's broadband infrastructure instead of a separate voice circuit. For a fuller explainer see what is TM MLS.
| TM PRI / E1 | TM MLS | |
|---|---|---|
| Delivery | Dedicated voice circuit | SIP over internet |
| Hardware at your end | PRI card / TDM gateway | SIP-capable PBX (or gateway) |
| Adding channels | Slow, line-bound | Faster, software-defined |
| Status with TM | Being retired | Current standard |
What stays the same
- Your numbers. Your TM DID range moves to the MLS trunk — no need to reprint cards or signage. See keeping your TM numbers.
- Your channel count. You provision the same number of concurrent calls (or more — it's easier to scale on SIP).
- The dial plan, mostly. Internal extensions and routing live in your PBX, not the trunk, so they carry over.
How the cut-over runs
- Order MLS with the same channel count and your existing number range flagged for port-over.
- Connect your phone system to the SIP trunk. A modern IP/cloud PBX registers natively; a legacy TDM PBX needs a SIP gateway.
- Test in parallel. Run inbound and outbound test calls on MLS while PRI is still live, so nothing drops on the day.
- Switch and decommission. TM moves the numbers to MLS; the old PRI circuit is released.
Why cloud is the better end-state
You can point MLS at your existing on-prem PBX and stop there. But if that PBX is the same ageing box that needed a PRI card, you're modernising the trunk while keeping the weakest link. Because MLS is just SIP, you can instead point it at a managed cloud PBX — keeping your numbers while gaining remote extensions, mobile apps, call recording and a system someone else maintains. That's the full path we cover in moving from TM MLS to cloud.
Don't migrate twice
The costly mistake is doing PRI→MLS now and PBX→cloud a year later — two projects, two cut-overs, two rounds of testing. If your hardware is near end-of-life, fold them into one move and you only disrupt the phones once.