Comcast is not a cellular provider in the traditional sense. They are a MVNO, they operate on the Verizon network. Most cable providers offer service thru the Verizon Network. Really the only Networks we have in the US are Verizon, AT&T, T Mobile and U.S. Cellular which I think has limited network coverage vs the big 3.
It was all good under John Legere, when he left it went to shit. Im planning on moving to Verizon. I can take my 3 lines and put them on my sisters account. Ends up being $27+ taxes per line, cheaper than what Im paying T Mobile.