There are several reasons a user might be unable to dial your USSD code.
It might be because the USSD code is a prepaid one. This means that the user must have sufficient airtime in order to dial it. In this case confirm that the user has airtime
There might be an issue with your callback URL.You may read more about this here: http://help.africastalking.com/ussd#callback-url-responses
Another reason could be if the Telco is experiencing downtime. In which case we are following up with the Telco to have them fix things.
If you have more questions, please send an email to firstname.lastname@example.org