The only place your text messages exist(ed) is on your phone and on the person who sent's phone.
If you backed them up, then they exist in your backups.
T-Mobile does not store any texts, contrary to TV crime stories.
However! You may be able to get detailed copies of your bills in the past which will tell you that a text conversation was happening between two numbers. If it's a question as to whether or not a certain number texted you, that record is available in your detailed bill. You can get the most recent 12 months on http://my.t-mobile.com and Customer Care can assist you with older at a fee. Unless, you are Prepaid, at which, there is no bill / records to get.
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