Iphone 8 Plus Problems

Bought a couple phones 12 months ago. Have 3 phones on the account and all worked well. My daughter has the Iphone 8 Plus and lives in an area outside Atlanta called Tate, Ga. Phone has always worked well until the last few weeks. All of a sudden she can't receive or make calls in her home. She can text and get on the internet but the phone service is terrible. Could it be a cell tower down or maybe get some kind of signal extender. Phone is useless in her house. Anyone have any ideas. Maybe new SIM card??