i haven't seen this around there.
i doubt that they are capable to do this yet.
I work in IT, we utilize VOIP on our network for our company.
To use a VOIP phone on Wi-Fi, you would have to remain connected to the network for it to work. If the resort's Wi-Fi signals only reach the boundaries of the resort, not sure what the point would be of the resort implementing this for VOIP.
Maybe if you have a laptop that you connect to their Wi-Fi network, you can use certain software that would route your calls to the VOIP phone. There are softwares that will allow you to make VOIP calls from your computer using a PC microphone/speakers (or mic/headset combo). Skype.com is one of them, and i think that they are free.
not sure i can help you much... but to be sure, call the resort you are looking to stay at (if not a disney resort), and ask them to find out through their IT department.
good luck