What phone are you using? When the issue does occur is it from one sender or multiple? If multiple do they also have T-mobile or are they on a another network and what device are they using? Sorry for all the questions but I want to see if there is a pattern that is going on when it occurs.
The situation seems to have been fixed.
But to answer your questions anyway, I'm using a Nexus 5.
It happened to multiple senders all not on T-Mobile.
The devices/services they were using was an iPhone 6, Google Voice, and CellTrust.