I'm on TMO and i haven't seen this. Are you sure it's not Text Forwarding? This is the first I've heard of this. Have you notified TMO and/or Apple about it? Even if limited, that's a pretty serious security issue.
Both. T-Mobile is aware of the issue. Apple may know if it, but may see it as an isolated issue, (which is particularly not). Some people know have said the same thing. So far, I have only heard these issues occur on iPhone 6's.