ATT, Tmobile and other gsm operators won't have this issue, calls will handoff from LTE, to HSUPA, to Edge etc.
Kind of... I do a lot of rural driving and go back and forth between LTE, 4G, Edge and even GPRS (although almost all of them are upgraded now). You still end up with a lot of dropped calls. I think part of the problem is that some of them are third party cell towers and the AT&T to partner hand off is not smooth.