If you're bonded to another network but want to switch to T-Mobile, don't worry: you have options.
Most of today's carriers communicate well with each other, and their technology allows you to cross over easily.
So if your phone was from Verizon, just ask them to unlock your phone for use on T-Mobile. After that, all that's left is to purchase a T-Mobile SIM card kit for only $20, and you're set.