AT&T's Voice Network

Can anyone confirm what AT&T uses for their voice network? My fiancee had garbage service with her iPhone 4 in the apartment (Greater Boston area), and we thought getting an iPhone 5s might help as it can use HSPA+ and LTE, but it's still dropping calls as much as ever. I'm only used to Verizon which never had these issues so I'm just trying to understand why AT&T is so terrible.

Thanks