SMS troubleshooting - delivery failures and reading DLRs

The delivery receipt (DLR) tells you where a message died. Reading it correctly is the difference between a one-touch ticket and a week of back-and-forth.

The common DLR outcomes, translated

• Delivered: the handset acknowledged it. Done.
• Absent subscriber: the handset was off or out of coverage; retries follow the destination's retry profile. A burst of these on one network can indicate a network outage on their side.
• Unknown subscriber / invalid number: the number does not exist on that network - stale data on your side, or a ported number your routing data has not caught up with.
• Barred / blocked: the subscriber or destination refuses this class of message - often sender-ID or content policy (see the sender ID guide).
• Expired: the validity period lapsed before delivery - the handset stayed unreachable through the retry window.

Patterns that matter more than single failures

• Failures concentrated on one destination network = network-side issue or filtering change; report it with volume numbers.
• Failures concentrated on one sender ID or content template = filtering; change one variable at a time to isolate which.
• Delivered-but-not-received complaints at scale raise the question of fake DLRs on a cheap route - tell us immediately; that is a fraud matter and we treat it like FAS on voice.

What to include in the ticket

Message IDs, submission timestamps with timezone, destination network(s), sender ID used, the DLR codes received, and volumes affected. Attach one full message example (content included) where filtering is suspected - we cannot diagnose content filtering blind.