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.