The gateway that was up, and unreachable

Symptom

Outbound SMS stopped arriving. The provider’s API was reachable by ping, its status page was green, and every other outbound request from the same machine worked normally. The TLS handshake simply hung.

False lead

Everything pointed at the provider blocking our IP. We were composing the support ticket when we checked plain HTTP to the same host — it answered instantly. A blocked address does not answer on port 80 and hang on 443.

Cause

A path-MTU black hole. The TCP connection opened on small packets, then the first large frame of the handshake was dropped somewhere upstream and the ICMP "fragmentation needed" reply never came back. Nothing timed out; it just stopped.

Fix

MSS clamped to 1360 on the host, applied as a service so it survives a reboot. Confirmed by bracketing the failure between 1400 and 1420 bytes before and after.

—When a service is "down", find a second thing about it that still works. The difference between the two is the diagnosis.