AT&T Iphone 4 cell service in my house all the sudden doesn't work?

Just the other day I stopped getting reception in my house. I used to get pretty good reception in my house and now I get nothing. And it's not like its a problem with at&t because my friend has at&t and while directly next to me he had 3 bars and I had nothing. I think it might have something to do with my cable or internet because the day this all started happening we installed xfinity cable and internet. If anybody has experienced this or knows something please let me know.

IT'S BEACUSE AT&T SUCK GO VERIZON DUH