Re: Nexus 6 on at&t
A few hours before you posted this, I got a call on my AT&T phone, so it's not AT&T. It could be AT&T locally, experiencing either problems or some VERY heavy usage. (Christmas isn't like Mother's Day - the highest number of phone calls on a single day in the US almost every year - but it's pretty high up there, maybe second place.) Call AT&T tech support (you can get the number on the web site and call from any phone) and see if they're having a problem in your area. They may have one and not even know it yet, because everyone is trying to call them on their cellphones - which aren't working.
And just in case anyone is experiencing any internet problems - delays, timeouts, etc. - from Christmas until school reopens after January 1st is about the highest internet usage short of the last few hours of April 15th and right after a major disaster. So if your internet is having problems, take a bottle of chill pills until January 2nd.