Hi, I'm Nathan.Sorry to hear that you're having trouble, but I'll do my best to help you. Please give me a few moments to review your details here.
Can you tell me a bit more about the problem please:1) If you have multiple handsets (within the same system), does this happen with all of them? 2) Are there other phones or even a fax, outside of this system but on the same line? If so are they working properly?3) Is this a landline from the local phone/cable company, or a VOIP service (vonage/magicjack/etc)?4) Do you have dsl service on the same line?
Okay can you start by unplugging power to your comcast modem/gateway, let it sit unplugged for 5 minutes, then replug, and give it about 5 more minutes to start back up, then test the ATT phones again and let me know the results.
Thank you for trying,
at this point it is still not possible from a diagnostic perspective to know if it is a comcast problem or an ATT problem. Both are very common, the only way to isolate this would be to test another phone, preferably just a basic corded or wall phone, on the line. Could you dig an old one out of storage, or borrow one from a neighbor for a few minutes to test?