iPhone calls ending instead of going to voicemail

phone rings 15 seconds but then disconnects the call. Caller hears messsge; ‘ your call cannot be connected at this time’

Happened when I Tried the *#61# to adjust the ring-time - it did adjust the ringtone fine but also has stopped the calls reaching my voicemail. Got this message:

“Setting Interrogation Failed

Voice Call Forwarding

When Unanswered…. “ etc

Carrier IDMOBILE can’t help as voicemail itself is working fine

iPhone 11 on latest software

have reset phone

have reset network settings

call forwarding is switched to OFF

any ideas out there please? Thanks for any help

[Re-Titled by Moderator]