Can other T-mobile customers call and message you? Did you bring your number over from T-mobile? It could be his line, or it could be that T-mobile's routing is wrong due to your number previously being on their network.
You may want to inquire to the person with t-mobile if they have any issues with texting everyone or just yourself. I would recommend having this person contact T-Mobile for troubleshooting dependent upon the answer you receive from this person on the texting issue.