Poor Mobile Phone Reception.

We have three phones on my account. Two work great in Metro Atlanta. One is with my daughter in Tate, Georgia. Matter of fact none of our phones work all that good at her house in Tate. When she has an important busness call she has to get in ger vehicle and drive 5 miles away to a place where she gets good service. Does Xfinity have an extender or reception enhancer I can purchase. She uses wifi calling and it does not do much better. Would increasing the wifi signal in her home help. The coverage map for cell towers is very sparse in Tate Georgia. Thanks for any ideas,