Welcome to our Support Community! I've searched high and low to find you an answer on why this is happening and came up with nothing. Does this error pop-up randomly or when you're trying to text? Also, is this generally in the same area or does location not play a part in this? Does this happen when you're connected to WiFi as well or only the T-Mobile network? I'd like to see if I can narrow down the root cause and go from there.
Is this still an issue? We know it's important to be able to text and we'd like to help. Have you tested the new SIM in another phone to see if it works?