From T-Mobile's terms & conditions:
"You must make a connection action at least once every 180 days. If you do not your SIM will be disconnected from the network and you will not be able to use the service at all. You will lose your mobile phone number. Any credit on your account at the time of such disconnection will be forfeited."
http://support.t-mobi..._TandCs_v1_online.pdf
All phone service providers have fairly similar conditions in their contracts. They need to be able to re-allocate unused phone numbers, otherwise they'd run out of numbers for their services. Further, the phones are usually sold to customers at much less than the cost of manufacturing them. Companies like T-Mobile expect to recoup their loss, and then make a profit, through the calls which are made. If calls cease to be made for any length of time, they simply take the remaining credit.
Chris