NEC DSX 80 -- Intercom call ringing an extension when it is in use

[LPNMRK]

Normally, when you try to intercom someone else in the office who is on their phone, you get "that extension is busy" or something like that. However, on one particular extension, it stopped doing that. Now, when you intercom that in-use extension, you hear it ringing and that person's phone rings even though they are on the phone. They must have pressed an option or other key and I can't figure out how to fix it. Can anyone help? Thanks.

 

[LPNMRK]

For anyone reading this, I figured out what was going on. The problem extension was designated as the operator for the system and the operator always gets off-hook signaling on a DSX.