When you send the airtime request successfully, you initially get a 'SENT' status.
This means that we have successfully sent the request to the Telco(e.g Safaricom, Airtel, MTN etc) but are yet to receive the final delivery report from them.
Once the Telco successful send the money to the phone number and we receive the delivery report the Status on the dashboard is changed to Success.
If you wish to get notified once the transaction is completed, check out this article :
Some times the transaction may fail at the Telco for various reasons. For further information send an email to firstname.lastname@example.org