Yoda:

Your post reminds me of one service call I had.years ago.
It is not exactly the same identical issue - but I think it is close enough to mention this.

I had an analog phone on a system, and weird things were happening when a call came in..

It took a bit of troubleshooting but what I found was the wall jack for the analog phone was bad.
It was in a dirty environment and the jack was dirty.
It was intermittently shorting with all that dirt and when that happened it showed busy and because of programming it would
forward calls coming in to some phone.that should not be getting incoming calls.

I replaced the jack and it fixed the problem.

Shorting the jack acts like it is off hook and with properly timed shorts maybe it is dialing 911 or the other extension.
I have never played/tested a jack by shorting the pins to see if it would dial.
But in the old days I could dial a phone if the dialer was bad or disabled by quick flash hooks.

Just a thought..

If CON is the opposite of PRO
Then what is the opposite of Progress?