If your contact is already up then you went have any termination fee, but you would need to pay the bill for your current usage. It would come after you finish your service. If you port your number out after your billing period has started, you'll be billed fir the full month.
I believe you'd only be billed pro-rata for the final month's bill. Billing for services not used could be fraudulent. That's what AT&T's done when I switched from them. I can't recall what T-mobile did in 2006 when I went back to AT&T, but I don't believe they charged me a full month's fees for a few days of service.
I work at Tmobile, so it's a question I've dealt with before. If you leave without advanced noticed, you're billed for the full cycle. It's only when you're in your 14 days buyers remorse period that it would be prorated.
I left T-Mobile a while back. If you're still under contract, they are going to charge you. Also, they are going to charge you for your current usage. T-Mobile will try to charge you for everything imaginable, even when it's their own fault.