If a 403 error is sent by the VoIP provider, it means that they are understanding your registration or call request but refuse to fulfil it. This could be for a number of reasons including:
- Incorrect username or password
- The number being called is not supported or you are not dialling the number correctly
- Lack of funds in the account
- You will need to reach out to the VoIP service provider to see why they are sending this error.