Service Access Denied is an error that is unique to a very small percentage of T-Mobile users.
We have done some research and identified that most times this can be solved in their phone settings.
Check the device to make sure premium messaging is enabled:
On some phones, it’s under Settings > Apps and Notifications > Special Access > Premium SMS Access. This will show a listing of apps that have requested Premium SMS Access - tap on the messaging app and choose "Always Allow".
If that fails, the member will have to reach out to T-Mobile Customer Service at 1-800-937-8997 and make sure they are able to receive premium messages (this is a free feature that is usually enabled by default). They should also make sure to have T-Mobile turn off “Device Block” if enabled.