1 ring then voicemail - solved

Was troubleshooting this for my mother, thought the fix could help others.

Basically the iPhone would ring once and then go straight to voicemail. Checked the carrier side settings and set call forward no reply to wait 30 seconds. This did not help.

The setting that seemed to be causing it was in Mobile > Calls on other devices. (In this in this case were 2 nearby iPads , or I guess whatever were signed in or had been signed into iCloud?)

Turned that off and presto - issue resolved!