As BREWERDAVE1 said its because you have not given T-Mobile 30 days notice
You are suppose to give 30 days notice in writing not just ring up for a pac code because you think your contract as ended
it don't work that way
what happens is when your 12-18-24mth contract finishes it don't as such it keeps going on a rolling 30 days contract and that is why you have to give the notice.

I wholeheartedly agree with you in principle ...but unfortunately no official body is willing to tackle what seems an unfair practice.
It always causes a vigorous debate on broadband forums when it comes up, because the same applies -you switch Internet Service Provider and still get hit with a notice period when you are being charged by both your previous and new broadband supplier.Also applies with a switch of landline supplier.
Some of the broadband suppliers have introduced much reduced notice periods -they all used to be at least a month - for instance, Plusnet who I am with, only expect 10 days notice now.
I'm not sure if the same applies with mobile phone contracts as I only use Pay as you go mobiles.

Thanks for your response Brewerdave. My point is that once the PAC code is used and my number gets transferred to a new provider I am not getting any service from T-Mobile so how can they charge me for a service they are not and cannot (since the number is no longer with them) provide?

If you look in your terms and conditions you will most likely find that you are liable to give a months notice of termination. T-Mobile will say that the notice period starts from when the PAC is used - seems reasonable when you consider that just supplying the PAC does not mean that you are actually switching.
The same principle applies for broadband connections on landlines,when a MAC is used.

I had a mobile phone contract with T-Mobile that expired in December 2011. I asked for a PAC code which T-Mobile gave me which expired on 09 January 2011. I was not able to transfer over the PAC code by the 9th of January 2012 and requested another PAC code which has now been given to me and which now expires on 08 Feb 2012.

I am now in the process of transferring over to my new provider (Vodafone). However T-Mobile are saying that I will be charged a full months line rental upto the 8th of Feb 2012 despite the fact that my nuumber will transfer over on the 12th of Jan 2012. Are they allowed to charge me despite the fact that the number on which they were providing me a service is not longer active on their network?