I have T-moblie service and have made calls from it at Disney. I have found that the places that I get the best service are: At the airport, just outside of the rooms (I have stayed at the Caribbean, All-Star, and Coronando Resorts and all is the same) and in the parks.
I am from MN, so I am not sure if that has anything to do with the coverage area?
HTH,
Korey