T46G - Line Call Forwarding to External #

[David R]

Equipment:
Yealink T46G - I have one phone, no PBX, Connected directly to modem, 5 incoming SIP lines.
Version:
Firmware Version 28.83.0.35
Hardware Version 28.1.0.128.0.0.0

Problem: Call forwarding is set for busy lines, and no answer to an external line. It stopped working this spring, and I have been unable to find the solution to properly troubleshoot.

What happens now:
The phone's screen says Forward to: [the programmed #], there is a brief busy signal, and the call is dropped.

What I tried:
I thought it might have been an issue with my virtual voice mail service, but it won't forward to any other number either. I have updated the software, I have reset the phone to factory specs, and I have followed the tutorials, and now have invested more time and energy into this than I care to admit.

Either the software has a glitch, or I missed something.

Please help me.