The company I worked for in the 2000s had both kinds of T1: They had a single CAS (channel-associated, or "robbed bit", signaling) T1 from one carrier, an two PRI (ISDN) T1s from another carrier. The two PRI T1s only had one channel for signaling, and 47 channels for voice calls.

Our block of phone numbers (we had a block of 500 or 1,000 numbers) was tied to the PRI T1s; for the CAS T1, we were allowed to use our number block as the "calling party" number for outgoing calls. If the PRI T1s went down folks wouldn't be able to call us, but we could call them and the called parties wouldn't notice any difference.