It is best not to keep on trying with the wrong number as the more they try and get it wrong the more likely it is that the card will be locked for good. Once it has been tried 10 or more times it will be locked for good and they will need to contact T mobile to have it replaced.
The first thing they will need is your PUK code. It will have been supplied to them in their SIM card wallet. If they no longer have that, or at least do not with them, they can arrange to get it online.
Either log on to the My T Mobile account or sign up for one. Once they are on pick out the phone from the list. Make the right one is chosen as any other phones on the account will have a different PUK number and they will not unlock the phone being used. Click onto "unblock sim” and then choose the option "if your pin code is locked”.
Your PUK number will come up and they will be able to put this number into the phone and unlock the sim. It will be a good idea to keep a record of this number just in case they do the same thing again.