well sprint doesnt support it i know that.. its usally not that the device wont, its more the carrier doesnt so the things you do to accomplish it are borderline illegal..
google voice allows you to set up a number, and assign it to upto 3 other devices... so people can call your TP2 on either the GV number or your VZW number.. you can also place calls,sms this way too.... you can technically accomplish zero minutes out and zero minutes in as far as your carrier is concerned using GV..
check it out and if you like it, i have an invite i can send you.. just PM me your email add if you like it and ill get ya invited
http://www.google.com/googlevoice/about.html