If you are attempting to send an SMS to one particular patient and it errors with no message, this means that the patient has opted out of SMS communications entirely.
How to opt out?
To opt out the patient simply replies CANCEL or STOP to any message they had been getting from the practice.
If a patient has taken this action the practice cannot override this.
If the practice attempt to contact the patient after they have opt-out, any message they attempt to send will error.
How to opt back in?
If a patient has opt-out by accident, and the patient wishes to be receiving messages, they can send START to the practice.
They should get a default message saying 'You have successfully been re-subscribed to messages from this number.'