Scott has an HTC One that he used at AT&T but he's had it unlocked and moved over to T-Mobile. But his MMS and text messaging aren't working now. Leo says that it may be that the APN settings are wrong, and that won't allow the phone to send images. There also may be an issue with the frequencies supported.
Leo says there's a YouTube video which talks about that very issue. It may be that the HTC One sold by AT&T is different in its capabilities than that same phone with T-Mobile. The chatroom suggests using WhatsApp instead. It bypasses the whole issue. But the other person he wants to text with needs to be using it as well.