We have a Private Line Automatic Ringdown circuit provided by the county government. Everything works well, except once in a blue moon, the phones will begin ringing as if a phone is coming in on the PLAR, when we pick up the reciever, the circuit connects and the dispatch center has no idea why we called them.
My theory is that something is causing the dahdi card to apply voltage to the line and make the line think someone is dialing out.
If the card is an FXO card it doesn’t have any way to provide voltage. If it is an FXS card then it would always have voltage ~48VDC. Ring is ~90VAC. so a DC voltage spike will not trigger a ring. Most firmware has debounce logic to filter out voltage anomalies.
Now if this is an FXO there is a chance there is some sort of wiring issue. Off-hook is when you apply a ~200 - ~400ohm load to the pair. So if you have something that is not a dead short but is applying that load then you are off hook then boom.
I used to see this when the line had a lot of phones on it. The voltage from the card would dip and the line would activate as if someone had picked up the phone. The one I remember the best was a customer that had an old-timey crank-to-ring phone that they liked to have “cuz it was cuul, dude!”
The ringer on it was about 500 ohms, so the line was always teetering on the edge of being “off hook”. Sneeze hard and the phone would pick up.
There are 6 things on the line:
2 Announcers that ring in the hallways when the line rings
1 Elevator phone
1 Outdoor Emergency Phone
The old PBX system is still connected, we have 1 phone on it for emergencies if the network goes down
The DADHI Card.
The interesting thing is that when this happens, ONLY the DADHI card activates and rings the VOIP phones. The elevator phone, outdoor phone, announcers, and old PBX, none of them will ring.
It’s caused a few problems.
There are boxes you can use to ameliorate this issue, since (as @dicko pointed out yesterday) the FXO ports are current-loop ports. Search the forum for “ring equivalence” from yesterday or the day before for more information and a good like to an Anixter (I think) box.