PRI, T1 and ISDN / to SIP
PRI to SIP migration without changing your numbers
A PRI carries 23 call paths over a T1 in North America, and 30 over an E1 elsewhere. SIP trunks carry the same calls over your internet connection, sized to what you actually use. We move the lines and keep the numbers.
Two paths
Keep the phone system, or replace it too
SIP trunks into your current system
If the phone system is staying, a gateway or session border controller connects it to SIP trunks. Handsets and call flows stay the same; only the outside lines change.
Hosted voice for the whole site
If the phone system is near its end too, we replace both at once: hosted voice, pre-configured phones and the numbers ported across.
What we check first
Where PRI moves go wrong, and how we plan around it
| Item | Why it matters |
|---|---|
| Every direct-dial number on the PRI | Blocks of numbers are easy to miss on a bill. A missed block stops ringing after the port. |
| How many calls you really make at once | Most sites use a fraction of 23 paths. Sizing to real peaks keeps the monthly cost honest. |
| Caller ID and outbound numbers | Each site should present the right number, signed by the carrier under STIR/SHAKEN in the US so calls are less likely to be flagged. |
| Emergency calling per location | A PRI often sends one address for a whole building or campus. SIP lets each site and floor carry its own. |
| Fax, alarm and modem lines on the PRI | These may need their own adapters or services, tested before the PRI is cancelled. |
| The internet connection | Voice needs priority on the firewall and a backup path. That is what our Connect plan adds. |
Security
SIP needs controls a PRI never did
A PRI is a private circuit. SIP runs over the internet, so it needs the controls NIST describes for voice over IP: accept signaling only from your carrier, encrypt where supported, block international and premium-rate calling you do not need, and cap the number of simultaneous calls so a compromised account cannot run up a large fraud bill overnight.
Questions
PRI and ISDN questions
Is our carrier retiring PRI?
Carriers are retiring copper-based services under FCC rules that require notice, and in the UK ISDN is part of the switch-off scheduled for 31 January 2027. Your carrier’s notices, or your next contract renewal, will usually tell you.
Can SIP fail over if the internet drops?
Yes. Calls can route to a backup connection, another site or mobile numbers. We design and test the failover path as part of the move.
Do we need new handsets?
Not if the phone system stays and connects through a gateway. Only a full move to hosted voice needs new or reprovisioned phones.
Related
Other legacy voice and network moves
Next step
Plan your move from PRI to SIP
Tell us your current carrier, how many PRIs or ISDN lines you have and how many sites they serve.