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.
. AT&T has a Next program where you can upgrade early but will pay full price for the phone in 12 or 24 month payment plan. I did this to get 6+ and took the one time payment option for the phone.