A PRI is a physical ISDN circuit with a fixed number of voice channels (23 on a T1 in North America, 30 on an E1 elsewhere), while a SIP trunk carries calls over an IP connection and scales by changing a setting. SIP trunks are usually cheaper and more flexible; a PRI gives predictable quality on a dedicated line but is being retired by many operators.
What is a PRI line?
PRI (Primary Rate Interface) is the ISDN service for businesses. It runs over a digital circuit and splits it into 64 kbps bearer channels (B channels) for calls plus one signalling channel (D channel) that uses the Q.931 protocol.
| Type | Where | Line rate | Channels |
|---|---|---|---|
| T1 PRI | North America (J1 in Japan) | 1.544 Mbps | 23 voice (B) + 1 signalling (D) |
| E1 PRI | Europe and most other regions | 2.048 Mbps | 30 voice (B) + 1 signalling (D) |
| ISDN BRI | Small sites | Two 64 kbps channels | 2 voice (B) + 1 signalling (D) |
Need 31 lines on E1? You order a second circuit and pay for 60. That step-by-step capacity is the main reason businesses move away from PRI.
SIP trunk vs PRI: what are the differences?
| SIP trunk | PRI (ISDN / E1 / T1) | |
|---|---|---|
| Connection | Internet or private IP link | Dedicated physical circuit |
| Capacity | Any number of channels, set in software | Blocks of 23 or 30 channels |
| Setup time | Hours to days | Usually weeks for a new circuit |
| Cost | Pay per channel or minute, international routes from many carriers | Monthly circuit fee plus calls from one operator |
| Voice quality | Depends on the IP link and codec | Fixed 64 kbps G.711, very stable |
| Resilience | Easy failover to another trunk or site | Tied to one building and one line |
| Equipment | IP PBX, SBC or softswitch | PBX with an ISDN card or a gateway |
| Future | Industry standard | Being withdrawn in many countries |
For how a SIP trunk works step by step, see what is SIP trunking.
Is ISDN being switched off?
In many countries, yes. Incumbent operators are moving their networks to all-IP and have withdrawn ISDN, or announced dates to do so. Timelines differ by country and operator and have changed more than once, so check your own provider's notice rather than relying on a general date. If your PBX only has ISDN ports, you can keep it for a while by adding a gateway that converts SIP to PRI, then replace it later.
Is a SIP trunk better than PRI?
For most businesses, yes: it costs less, scales by settings, supports failover between sites and lets you buy international routes from carriers that specialise in each destination. A PRI still makes sense in a few cases:
- The site has no reliable broadband or private IP link.
- Old equipment (alarm panels, lifts, fax) needs a circuit-switched line and cannot be replaced yet.
- A regulated environment requires a specific certified connection.
Quality on SIP is excellent when the link is sized and prioritised correctly: keep jitter under 30 ms and packet loss under 1% (see VoIP jitter and latency) and choose a codec that fits your bandwidth (see G.711 vs G.729).
How do you migrate from PRI to SIP trunking?
- Count your real peak calls from PBX reports. Many sites with two E1s (60 channels) never use more than 25 at once (see SIP trunk channels).
- Check the PBX: native SIP support, or a SIP to ISDN gateway in front of it.
- Plan number porting of your DIDs to the new provider (see DID numbers).
- Secure the edge with a firewall or session border controller.
- Test outbound, inbound, fax and DTMF, then move traffic in stages with the PRI as a fallback (see how to test a SIP trunk).
Carriers that still interconnect with TDM (E1, SS7) do so for some wholesale destinations; our page on TDM vs VoIP routes explains when that matters for buyers.
SIP routes for your traffic
Once your system speaks SIP, you can buy routes per destination from wholesale carriers instead of one operator. VoIPMatcher sources and manages those VoIP termination routes for you. Request a SIP trunk route.
Frequently asked questions
How many channels does a PRI have?
A T1 PRI, used in North America, has 23 voice channels and one signalling channel. An E1 PRI, used in Europe and most other regions, has 30 voice channels and one signalling channel, plus a framing timeslot. Each voice channel carries one call at 64 kbps, so capacity grows only in whole circuits.
What is the difference between SIP trunk and PSTN?
The PSTN is the public switched telephone network, the global system that connects all phone numbers. A SIP trunk is a way to reach the PSTN over an IP connection. Your provider converts your SIP calls to the format of each destination network, so callers on ordinary phones do not notice the difference.
Can I connect an ISDN PBX to a SIP trunk?
Yes, with a VoIP gateway. The gateway has ISDN ports on one side, which the PBX sees as a normal PRI or BRI, and an IP port on the other side that registers or peers with the SIP trunk. This lets you keep an older PBX working while the operator retires ISDN.
Is E1 the same as PRI?
Not exactly. E1 is the 2.048 Mbps digital line with 32 timeslots. PRI is the ISDN service that runs on it, using 30 timeslots for calls and one for Q.931 signalling. An E1 can also carry other signalling, such as SS7 between carriers or older channel-associated signalling, so every E1 PRI is an E1 but not every E1 is a PRI.
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