I have used my tracphone at WDW with no problems. Cell service is pretty spotty around the parks so you may have to walk around to get a good signal.
I didn't bother with changing my local calling area to WDW beacuse I didn't use it much, just to call home and check on things and to call a friend who was also in the park a couple of times. I just accepted the 2 unit/minute charge for roaming, still cheaper than calling long distance.