Hehe, 20 minutes on hold? Smart answer: get a different mobile phone provider.
Anyhow, as long as the phone isn't locked for another provider, it should work fine . . . unless Nextel does something strange to make it not work if the IMEI isn't recognized by the network, though I don't know why they would. I sure as heck wouldn't buy a phone before hearing from them though. And I don't have Nextel, but unless it's something that you have included with your rate plan, you can be pretty sure you'll be charged for it, and that includes test messaging. I would guess anywhere from $.05 to $.10/msg.