Follow the given procedure to resend failed messages:

• Under SMS History object, check the Sent Status field for the SMS.
• If the sent status of the SMS displays Failed, select all such failed SMS from the list view.
• Click Resend SMS list button to resend these SMS.

 If SMS delivery fails due to insufficient SMS credits, customers can recharge their SMS-Magic account and raise a case to resend failed SMS. Our Customer Success team can help you in the process if you provide them with the following details –

•  The preferred date range for sending the SMS
• Your account ID.