No national carrier has truly ever completed replacing AMPS in all areas of their network like they should have. AT&T Wireless had AMPS & TDMA coverage on CA127 on the road from Baker, CA to Shoshone, CA. After AMPS was shuttered, they never lit up GSM to fully replace that coverage area. They only covered what they thought would be the areas that would bring them profit. Sad too how they pulled that one off.
If Sprint ever decided to go the GSM route, at least there is already nationwide GSM deployment, but even then, LTE will fully cover this country before that would ever feasibly happen.