Just get the 3 Sim Only contract, and give your cancellation notice immediately. We just did this, it cost £20 for loads of minutes and 12gb of data, all of which can be used in the USA with no extra cost to call either within the States, or back home. My wifes phone did keep going on T-Mobile which was annoying, but you can usually manually select a network abroad, so choose AT&T as their coverage was perfect in the whole of Orlando.
Whatsapp, Facetime or other voip calling is ok if the Wifi in Disney is stable at the time you want to make a call, but can be patchy if the park is busy. If this is coverage for a job or emergency, I'd suggest the 3 route