I have done this when i took my Tmobile sensation 4g when i went to india for vacation.
Just go to the store or call the customer service and give the same reason. They will send the unlock codes via sms or by mail. Im still under contract with Tmobile (only 6 months completed) but my phone is unlocked. Was using Airtel 3g when i was in india for a month.
Try it. Im sure they will help you with it. All the best for your trip