I found a ringback number that works. I'd like to add it as an answer.
Please reopen Verizon ringback number for testing Caller ID?.
POTS and PSTN telephone systems are not computer hardware or software, they are public telephone infrastructure and have nothing to do with your computer hardware or software. They are also outside of your control and subject to the whims of the telephone operator.
The clause in the help that most closely matches the type of question you have is
personal and home computer networking
But it fails the "personal and home" part because your question is about a public telephone network that has little to do with your home except for a short length of wire with nothing except a suspected good modem that isn't actually at issue.
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 using computer signals (via a modem) 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. This is a telephone problem, not a computer one. Until you can prove that the line is okay and that the modem itself is the problem then the issue is not with computer hardware or software.
Super User is not a telephone directory service.
To quote the close reason on the question:
Questions seeking product, service, or learning material recommendations are off-topic because they become outdated quickly and attract opinion-based answers.
This seems like a straight forward case of this close-reason - asking for such a number will soon become outdated, as evident by the resources linked form the question that contain such outdated numbers.