Visitors might not be receiving SMS messages because:-
There is no more SMS credit available
Below are the steps to check the SMS credits:
Go to the "Reports".
Click on the "SMS transactions".
From there, you can check the balance.
The Contact number is not valid. In that case, SMS credits will be deducted from the account.
Below are the steps to check whether the contact number is valid or not:
Go to the "Reports".
Click on the "SMS transaction".
3. Now, you can check if that number is invalid or not.