With a 3CX 16 SP5 installation, all calls to a user were being diverted to voicemail. Verbose logging showed that the system thought the extension had DND enabled, but the Yealink T42S phone did not have DND on. The PBX acted like DND was enabled when it wasn’t.
Long story short, 3CX seems to have a legacy DND status that is kept on the server and not shown in any UI. it’s set by dial codes:
*60 Disable DND
*61 Enable DND
As of this writing, these codes are missing from the official Dial Codes documentation:
https://www.3cx.com/docs/pbx-dial-codes/
The fact that this is a server-side setting probably explains why a packet trace didn’t show any SIP INVITEs going to the phone. This Yealink article is correct to point to a server issue if no INVITEs are sent.
In this case, the user had tried to use the U.S. standard *67 to block outbound Caller ID and must have accidentally dialed *61. Once I dialed *60 from her phone, calls started arriving again.
By the way, to block outbound Caller ID on a call through 3CX, dial *5 before the number. That is in the Dial Codes doc.