Ring Group Forward to Outside Number Failed

[idscan.net]

Some info about our setup:

• 3CX Version, Professional Perpetual 16.0.5.612
• Server OS, Windows Server 2016
• Provisioning Method: Stun
• Has the Firewall Checker passed: YES
• Are custom Phone Templates being used: NO

We have a 3CX server that was running fine a few weeks ago, but now our forwarding doesn't work when a ring group doesn't answer.
There's an answering service that takes messages for us, and calling the number directly works.

When the last person in the ring group does not answer, you hear the following message:
"Call transfer failed: no answer"

This has worked for months and now suddenly it stops.
Any advice on getting this set up again?

Thank you.

 

[leejor]

....and what does the 3CX Activity Log show for the call attempt? Was it even sent to the provider? If so, was it rejected? Was there a SIP message returned?