I think there are two things to consider -- the specific cell phone, and the carrier. I believe the carrier for Straight Talk is Verizon. You really need to ask about this at the place where you would purchase, then find out if Verizon has good coverage where you live (and where you will want to call). We are very rural and Verizon service out here is not very good; it could be completely different where you live.
T-Mobile is notorious for dropped calls. However, my dd's problem with her phone cutting off and turning off turned out to be her phone. When I got her an iPhone, all of those problems stopped.