Yealink T53w not provisioning or registering

[SteveITS]

When it is on the 192.168.1.xxx network what is Outbound Proxy set to? It should be itself.

Does this phone connect on any other external network or is the problem specific to one network?

I keep thinking "factory reset and start over" but you said you did that already.

 

[ash720]

It now works!
Newest version of 3CX installed and a newer firmware version for the phone became available since I started this topic. Maybe those solved the issue.
The only other thing I did differently was follow the advice on another thread of factory reset rather than simply restarting the phone post-installation.