Straight Talk Trackphone sold phone but have no signal.was told that they just sell service and don't guarantee a connection, Internet

Purchased a new phone at walmart and service thru straight talk.service was sprint Have no signal at home.called them and told them and i wanted to trade it for a phone with another carrier and was told that they could not do that.i complained about not having service at home and they told me that they just sell the phone and service but don't guarantee service they would not let me trade for a phone with a different carrier.wanted to go with a verizon phone for they have good service in my area.