I'm not sure how T-Mobile works, but the only time I've considered suspending service was when I fell behind on payments with Rogers (this was about 2 years ago). I was told my account had to be up-to-date, and I would still be billed $15/line.
So, that's 2 year old info on a totally different carrier, for what it's worth. Contact customer care and inquire about suspending service and any costs involved in it - keep in mind if you're on contract you will likely still have to pay them something in order to keep your account in good standing, and even if you're not, I doubt they will keep your account active for free. I would imagine they will have some sort of fee to keep the account active. Best thing would be to contact them and inquire, but don't make any mention of your intentions of leaving just yet.