Below are the main reasons this may happen:
Trying to send airtime to a number that had a successful airtime transaction in the last 5 minutes. Please wait for 5 minutes to elapse before sending more airtime to the same number.
Sending Airtime to an invalid number. Confirm with the client that the number is active and in use.
Insufficient balance: This just means your Africa's Talking account balance has been depleted. You can find Top Up instructions here: http://help.africastalking.com/website/how-to-top-up-your-account
Network connectivity issues: Sometimes Telcos have connectivity problems when this happens we send Service Advisories to all our airtime clients.
If these reasons don't match your transaction failure, kindly send an email to firstname.lastname@example.org with the phone number and time of the transaction then we will raise it with our service providers