There are a number of reasons a callback URL might not be receiving POST parameters from Africa's Talking.
Here is a checklist you may use to debug:
Ensure your code is correct. Incorrect code sometimes causes your callback Url to return a Status code 500: Internal Server Error. You may use tools such as Postman to test your Callback URL and ensure it works correctly.
Status code 403:Forbidden. This occurs when we try to invoke your callback and were denied access. In this case you should check the permissions on your callback URL file. If that doesn't resolve things try getting in touch with your hosting provider to have them whitelist our IP address.
Status code 405: Method not allowed. This just means that the POST method is not allowed by your callback. You may rectify this by enabling the POST method.
General SSL Engine problem: This occurs when the SSL handshake between our servers and your callback URL fails. This can be fixed by checking the validity of your SSL certificate here.
If this none of the above resolved the issue please reach out to us : firstname.lastname@example.org