All Delivery Receipts with a non-zero error code can be considered as indicating failed delivery. i.e. Your SMS did not reach the recipient.
Neither Nexmo SMS APIs nor operators will perform further attempts to re-deliver a message unless another API call is performed.
Nexmo error codes:
|2||Absent Subscriber - Temporary|
|3||Absent Subscriber - Permanent|
|4||Call barred by user|
|13||Subscriber Age Restriction|
|14||Number Blocked By Carrier|
|15||Pre-Paid Insufficient Funds|
For further information see: Why was my SMS not delivered?