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:
| Error | Meaning |
|---|---|
| 0 | Delivered |
| 1 | Unknown |
| 2 | Absent Subscriber - Temporary |
| 3 | Absent Subscriber - Permanent |
| 4 | Call barred by user |
| 5 | Portability Error |
| 6 | Anti-Spam Rejection |
| 7 | Handset Busy |
| 8 | Network Error |
| 9 | Illegal Number |
| 10 | Invalid Message |
| 11 | Unroutable |
| 12 | Destination Un-Reachable |
| 13 | Subscriber Age Restriction |
| 14 | Number Blocked By Carrier |
| 15 | Pre-Paid Insufficient Funds |
| 99 | General Error |
For further information see: Why was my SMS not delivered?