Two phone calls and a visit to local Cricket office have failed to fix this issue which has surfaced in recent months after more than a year of satisfactory service.
Sending via MMS is unreliable. Rarely sends on the first attempt. But apparently I can receive OK.
Internet searches suggest the problem is DNS returning an MMS server IP address which is not reachable via WiFi and the public internet. Suggests an override setting for Android which is not possible on iOS. But that provides a hint which usually works: disable WiFi on my iPhone, reboot, and then it seems to reliably send MMS for a few days. But recently has starting having trouble sending SMS too. But no problem messaging to Apple users on iCloud.
Would think an unreachable IP address would result in an instant failure, but the failure message takes minutes. Green bar appears across the top of screen, reaches about 95% then hangs for minutes until the error message. This seems to be more of a timeout at the server than unreachable IP. Trying to send text and picture in one message fails 100%, as does sending two pictures.
Best answer by BillyM_CricketView original