Postpaid? You can log into http://my.t-mobile.com and see the ACTIVITY (e.g. a text in from 404-555-1212 at 4:12pm on 1/4/2016) for up to a year.
If you're looking for the CONTENTS, that is not stored anywhere except your phone. Unless you're backing up the texts from your phone, they're gone if the phone is gone.
Unfortunately as per policy via FCC I dont think they have records of it. They don't have the options to stored personal messages