said by Hagar:
Initiate the number port with your new provider after the tmobile contract is up but the account is still active.
This part is critical, if the MDN is not active it will be rejected. The kicker for TMobile retail is they respond to the 1st port request immediately, but if it fails and then the request is resubmitted.. it gets put in another bucket and can take a few hours before it is looked at. So make sure all information is correct before submitting, this will make the port a smooth process.