You can move from PRI to SIP without moving the PBX to the cloud
Replacing a Primary Rate Interface (PRI) with a SIP trunk does not require moving the PBX into the cloud. The carrier connection and the location of the call-control system are separate design decisions.
An existing on-premises PBX may accept SIP natively. If it does not, a voice gateway can present PRI to the PBX while connecting to the carrier over SIP. That gateway creates a migration boundary: the PBX continues to see a familiar circuit while the external service changes.
Three migration shapes
| Approach | What changes | Best fit |
|---|---|---|
| Native SIP trunk to the existing PBX | Carrier interface and PBX trunk configuration | Supported PBX with current SIP interoperability |
| SIP-to-PRI gateway | Carrier side and a new gateway; PBX retains PRI | Stable legacy PBX that cannot accept SIP safely |
| New cloud PBX and SIP service | Carrier, call control, endpoints and operations | A broader replacement with its own business case |
The third option may eventually be right. Combining it with a forced carrier migration, however, increases the number of moving parts and makes rollback harder.
Inventory what the PRI carries
Do not reduce the old service to a channel count. Record the number ranges, inbound digit presentation, outgoing caller identity, diversion information, fax, modem or alarm use, emergency-call treatment, call-by-call routing, busy behaviour and any QSIG or proprietary features.
A gateway can translate signalling, but it cannot guarantee that every supplementary service has an equivalent representation. Test the features the organisation actually uses.
Keep rollback physically possible
Where contracts and number routing permit, run the new SIP path before removing the PRI. Use temporary numbers or a subset of traffic to prove:
- inbound number delivery and formatting;
- outbound identity and permissions;
- codec and DTMF behaviour;
- fax or analogue dependencies;
- concurrent-call limits;
- failover when internet, gateway or carrier access fails; and
- emergency calling with the correct location and callback number.
Do not let the gateway become invisible
A SIP-to-PRI gateway buys time, not freedom from operations. Monitor both sides, back up its configuration, restrict management access, maintain supported software and document its clocking and cabling. Fault isolation now includes a translation boundary that did not exist before.
Separate the urgent access change from the strategic PBX decision. A controlled SIP conversion can preserve the current phone system while the organisation evaluates cloud, on-premises and hybrid call control on its own timetable.
Related: SIP trunking fundamentals, VoIP migration planning and the deployment checklist.
Technical references
Scheduled for 21 August 2026. Confirm emergency-calling, cabling and carrier requirements for the deployment jurisdiction.