I have a Tadiran system. This morning, I got a complaint that one of our fax lines gives a busy signal every time they try to send a fax. When I dial that extension from my phone, the caller ID display says "Locked Out." In the FEAT of the extension, it says:
HOT_ST_IMM DEST = 9
PRIVACY ASSIG
Any ideas what is causing this behavior?

Unplug the fax machine and then plug it back in.

Thanks for the reply - I did get it fixed last night. I finally got a chance to run down there with a phone, and found the port was completely dead. My cabling guy came out, and found the port was bad. We've had a bunch of strange issues regarding our phone switch, so I initially thought that was the cause.

Sounds like you have a good handle on things. Welcome to the board. If you have more questions about Tadiran, please don't hesitate to post. There are a lot of knowledgable people here.

Thanks... I'm going to need all the help I can get. I've worked on the data side for over a decade, but the voice stuff is fairly new to me.

This has happened to me two nights in a row, the only difference being that the fax machine works OK after it's unplugged and plugged back in. When we arrive in the morning the port is "locked out." Are there any causes other than a failing port? System has been in place almost two years, this just started happening a couple of days ago. Thanks!!

The most common way to get into "locked out" is to leave the handset off-hook for long time.
The same will be if you have a bad cable/wire with short between the 2 wires.
Check with another buttset or phone.
If you have no dial tone or noisy line check near PBX.