Caller ID abruptly quit working

I just noticed this morning when my husband called. We have a VoIP land line attached to our Uverse internet, and for the last couple days, there has been no Caller ID identification of incoming calls. I only noticed because we leave the ringers off, and normally the answering machine will give out the incoming phone number, but the first thing I heard was our outgoing message. (The point of hearing the incoming number is to be able to pick up if it is one of the family.) The Caller ID information also did not display on the Caller ID machine we have attached to the line, one we bought when we moved into the house nine years ago. Yes, it is an AT&T machine, a CL2909 phone-plus-caller ID device. The last display number was two days ago, and there are only 28 numbers in history at the moment (I think it stores up to 50). I thought Caller ID was a default part of the phone service, but I don't know how to turn it back on and we need it to be on. Any suggestions would be deeply appreciated.