If T-Mobile is the carrier you are subscribed to and you experience this issue, try calling them on 1-877-453-1304 if you are in the US, or 1-505-998-3793 if you are out of the US, at the moment.
If you didn't manage to call them and if you have access to a computer, visit T-Mobile's troubleshooting page, which will try to diagnose and fix this issue.
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