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I can't find anything in the official documentation for Change of Responsibility:
I learned yesterday through their fourms that AT&T's policy for change of responsibility apparently does a proration since the number was already paid for to AT&T up to a specific date (this does not apply to porting). I'd like to think the same happens to T-Mobile plans. Especially since I don't think I recall seeing a complaint in the forums of someone doing a CoR and double-paying for the overlap period (there's a lot complaining about not prorating with a port).
I would wait for one of the T-Mobile mods to chime in, though.
A lot is going to depend on the plan you were on and if you had any promotions on your specific line as well as if you were on bill current billing or bill in arrears. We will need to take a look at the account in order to give you the correct answer here. Please call us when you have a moment so our care folks can assess your bill.