The community manager offered info in the thread below. You may have to contact T-mobile to have them block the messages if they become to much of an annoyance.
https://community.t-mobile.com/tv%2Dhome%2Dinternet%2D7/spamtext%2Dmessages%2Dare%2Ddisplaying%2Don%2Dmy%2Dmodem%2Dhow%2Ddo%2Di%2Dstop%2Dthis%2D44451