When TM moves you to MLS but you still have a perfectly working Avaya, NEC, Panasonic or Ericsson PBX, you don't have to scrap it on day one. A SIP gateway bridges the old world and the new: it converts your legacy PBX's PRI or analogue ports into SIP so the system keeps running over a modern trunk. Here's how it works, when it's the right call, and when you should skip the gateway and go straight to cloud.
What a SIP gateway actually does
A legacy PBX speaks TDM — E1/PRI digital circuits, or FXS/FXO analogue ports. A SIP trunk like MLS speaks SIP over IP. A SIP gateway (also called a VoIP gateway) sits between them and translates: PRI/analogue on one side, SIP on the other. To your old PBX it looks like the phone line it always had; to the MLS trunk it looks like a normal SIP endpoint.
Which gateway you need
| Your PBX connects via | Gateway type | Typical use |
|---|---|---|
| E1 / PRI (digital) | PRI–SIP (E1) gateway | Mid/large PBX with a PRI card |
| Analogue extensions | FXS gateway | Old analogue handsets/devices |
| Analogue trunk ports | FXO gateway | Small PBX with line ports |
The right unit depends on your port type and channel count. Sizing it is part of scoping the migration — the wider path is in migrating from TM PRI to MLS.
How the connection is set up
- Identify the ports. Confirm whether the PBX presents PRI, FXS or FXO, and how many channels.
- Place the gateway. It's cabled to the PBX's existing ports on one side and your network on the other.
- Register to the trunk. The gateway authenticates to the MLS (or cloud) SIP trunk and maps your numbers.
- Test inbound & outbound in parallel before cutting over, so the old PBX keeps answering throughout.
Gateway vs going straight to cloud
A gateway is a sensible bridge — it protects a recent investment and buys time. But be honest about the maths: if the PBX is near end-of-life, you'd be spending on a gateway to keep alive a box you'll replace soon anyway, and still missing remote extensions, mobile apps and built-in call recording. In that case it's usually better value to point MLS straight at a managed cloud PBX and retire the old hardware. We weigh both options in moving from TM MLS to cloud.
- Use a gateway when the PBX is fairly new, fully paid off, and meets your needs.
- Go straight to cloud when the hardware is ageing, you have remote/branch staff, or you want features the old box can't do.