The only network that allows me to make a call wherever I want, whenever I want, appears to be Verizon.

There's really no substitute for availing yourself of the provider's free trial period, which is usually two weeks.

You can use this trial period to make sure the network and the phone work everywhere you need for it to work.

It's a pain, but it beats being stuck in a contract when you decide later that you need to be on a competing network.