We have several BRIs to provide a specialized service, which requires a specific type of hardware per BRI. We're struggling with offering incoming calls, as each of these lines has its own telephone number and we were told by the Telco that this is how it works, and each line can only have one "SPID". We did however ask for numbers, not SPIDs, but their answer suggests that the two are linked.

It is possible and common in Europe to assign a single phone number to multiple lines, ISDN or not, on a first answerer gets the call basis.

Is a North American ISDN line indeed bound to its SPIDs in a way that it is limited exclusively to receiving calls from a single telephone number per B-Channel, and if so, must that number indeed be unique? Or is it possible to configure a number for ringing on multiple BRIs?


Each B-channel has a SPID (circuit ID). It doesn't have to have anything at all to do with the PSTN phone number. Many LECs in the US assign the two together, but nothing in the switch requires this.

Individual lines can be mapped to a "hunt group" with a single pilot number, and/or each line can be programmed to roll forward to the next ("rotary").

(Good luck finding anyone at the telco who knows how any of this stuff works. When you do find them, get their direct phone number!)


In Europe, ISDN became popular, but in the U.S. it never really took off the same way. The problem in the U.S. is that each carrier does it differently, and, even worse, it is handled differently in different states.

For instance, in most U.S. locations, ISDN was billed by per minute of usage, not flat-rate as is common for regular phone lines. In my state the state government wanted ISDN, and it really force the main carrier in the state to offer it at a flat rate.

In the U.S., you are, unfortunately, at the mercy of the incumbent carrier at the location, and the same carrier may have different rules for different states. The only long distance carrier that could even transport ISDN calls was AT&T, which is no longer even a stand-alone entity/LD carrier.

A SPID is the circuit ID, and it does directly relate to the phone number. Usually, a SPID starts with the 10-digit phone number with something like 0101 tacked on the end.

Back when I got an ISDN BRI for personal use (before the current DSL and cable internet connections), I used it for both data and phone service. Calling one of the two phone numbers assigned to the BRI would ring that line, but if that line was in use, it would ring the other line. To get that to work across multiple BRI lines would involve the carrier to get that enabled. Unfortunately for you, there is no way to force a carrier to do that, and, in many states, they can't offer such a service without getting a tariff granted by the state to offer the service. At this point, ISDN BRI never really took off, and it isn't cost effective for the carriers to jump through the necessary hoops to add such services. The carriers are more concerned about getting all their services on IP, and off TDM (like ISDN), rather than enhancing and adding TDM services.

  • There's plenty of ISDN PRI in the US. BRI was never all that popular (mostly due to cost)
    – Ricky
    Oct 28 '15 at 19:09
  • For most of the U.S. that's true, but we eventually dumped BRI because there were just so many places we just couldn't get it; we end up dealing with a lot of Mom 'n Pop incumbent carriers who will offer PRI, but not BRI. Here, in Texas, it was actually cheaper ($47/month flat rate) than two standard phone lines with the extra features.
    – Ron Maupin
    Oct 28 '15 at 20:09
  • @RickyBeam, what I actually meant was that ISDN BRI was popular in Europe for use as standard residential phone lines, so it was everywhere, but it never took off that way in the U.S. We are now trying to dump PRI, because of cost, in favor of more centralized SIP trunking, but we still have problems with that where Mom 'n Pop are the incumbent carriers.
    – Ron Maupin
    Oct 28 '15 at 20:34

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