We observed an increase in filtering on Toll-Free SMS to the T-Mobile network starting September 12, 2018. Messages to T-Mobile are sometimes filtered and counted as spam.
This is especially true if your text messages contain:
2.Link (Confirmation links)
Your users that are using T-Mobile and want to opt out of this filtering can contact T-Mobile customer care, and request that T-Mobile remove the block on Toll-Free / business messaging from their account.