This is T-Mobile. I had my cellphone number switched to BOOST at the end of my contract and they call it regularly to collect about $47 from my final bill. I'll get around to paying it one day :).
Make sure if you cancel your account to get the name of the representive, the time and date of the phone call, and a confirmation number. If you don't the will try to cheat the system after a few months and say you owe some huge amount, and you will not have any proof to the contrary. Happened to me.