Hmm, that usually means that you have no cell signal and wifi calling is off or your SIM card isn't working right.
Try removing your SIM, wipe down the contacts just for fun, and put it back in. See if that helps.
You could go to a TMo store and see if they'll give you a new SIM if it keeps doing that.
Check your APN and connection settings (on mine it is in Settings > More under networks > Mobile Networks). Make sure Network Mode is set to GSM/WCDMA/LTE auto, make sure your Access Point Names is set to T-Mobile LTE, tap Network Operators > Search Networks and choose TMo. Maybe something there will clear it up.
After that I guess you could try a Factory Reset but you'll lose anything that isn't backed up.
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