Help full signal, can't make or receive calls or texts.

[seannemairi]

ok, so i've had this phone since last october but it just started doing this in january. It only happens in certain places (anywhere in pennsylvania, washington dc, among a few other places.) I will have a full signal and will be able to use data but i can't make or receive phone calls or send or receive texts. I am a flight attendant and it's incredibly frustrating and inconvenient. I've gone to the at&t store and all they did for me was give me a new sim card which didn't work and isn't the issue. I haven't made any alterations to my phone like rooting etc mostly because i don't know what that is or what it does. The only time it does work in those places is when i'm in philly and my data icon has an E instead of an H. that's when i know i have a short window to make calls etc. also sometimes it'll give me a sim #3 error or something along those lines but it doesn't last long and doesn't happen often. can anyone help a non techie out? i'm beyond frustrated. thanks!