Phone Doesn't Ring with Some Incoming Calls

On a number of occasions, our telephone doesn't ring with an incoming phone call. The vast majority do but we were waiting for some important call-backs and the call came in but went to voice mail and did not ring. Is there something wrong with my system. I tried to called and Xfinity rep but there are no phone numbers, just Xfinity Assistant which was worthless. I need to get this resolved because the calls that have done is are important.

Thanks.