POTS and PSTN telephone systems are not computer hardware or software.  They are also outside of your control and subject to the whims of the telephone operator. 

If your problem was actually connecting to the network in the first place using a modem then it could be on topic. Your problem is *only* that you need a "callback number" from a telephone provider so that you can read a caller id number. 

If you *had* gotten a callback but your modem was failing to get caller ID then that *might* be on topic if the problem was with your modem and not the telephone operator. This would be tested using another known good device.

You might be subject to operating computer signals for working over their network but your problem doesn't even involve sending signals across the network except to dial a number and get a call back.

Super User is not a telephone directory service.