[VoiceOps] False 911 calls and old abandoned DID

Mike Johnston mjohnston at wiktel.com
Thu Jan 21 18:07:51 EST 2021

Some of my equipment will show a notice, hinting at a possible issue, 
when "MORE THAN 10 DIAL PULSES WERE RECEIVED".  A couple years ago, the 
responsibility of our switching gear was being transitioned to me as the 
previous guy was retiring.  During that period of time, I noticed a 
station in particular would throw that message a lot, up to 50 times a day.

There is also an entry every time somebody calls 911.

One late night I saw somebody call 911 multiple times.  I looked up the 
line and found it was the one with all the MORE THAN 10 DIAL PULSES WERE 
RECEIVED issues.  I called the sheriff's office. I asked if they had any 
weird 911 calls from that number.  They replied, ecstatically, "YES, 
like seven times!"  They said it would be static for a long period of 
time and eventually hang up. I suppose the dispatchers really shouldn't 
be quick to hang up their end, just in case.  So they have to wait out 
the call...listening to the static.

I promptly unwired it and put in a trouble.  I thought, surely this 
isn't a common issue, right?  Lines with static on them don't just call 
911 by themselves!  Do they?  Well, in my phone call with the Sheriff's 
Office, they said this is NOT an out of the ordinary event for them.

I audited some more.  One of them, with a butt set in monitor mode, I 
heard a lot static with, "If you'd like to make a call please hang up 
and try again," in the background.  Looking at our logs, that one had 
called 911 too.  We keep those logs for 6 months.  In that 6 month 
period, my analysis found 16 "left-in" stations that had presumably 
self-dialed 911 a total of 35 times.

Since I stumbled across that, we keep a much closer eye on these sorts 
of things.  Basically, any line that frequently or repeatably throws the 
MORE THAN 10 DIAL PULSES WERE RECEIVED notice, we do an audit on.

A little more background on what causes this:

Pulse dialing is performed by rapidly interrupting the telephone circuit 
for each digit dialed.  This method was sometimes referred to as loop 
disconnect dialing.  When you think of pulse dialing, you might 
immediately think of rotary phones.  You might also have visions of 
stepping switches.

One interrupt represents the digit "1" being dialed.  Two rapid 
interrupts represents the digit "2", and so on.  You need to have at 
least a brief timeout between the digits to indicate you are done with 
that digit and starting the next.  The exact timing is relatively loose, 
leaving a lot of wiggle room.  Any sort of interference that is creating 
a make/break cycle has the potential to pulse dial.  Multiple makes and 
breaks with the right timing (and again, the timing is pretty loose) 
could conceivably call anywhere...with no human being involved!  The odd 
thing isn't having it happen - it's having it happen to a specific number.

For 911, you simply need:
   - 9 rapid interruptions
   - pause
   - 1 interruption
   - pause
   - 1 interruption

For lines that are shorting out, arcing, in water, or whatever, it is 
only a matter of time.

-Mike Johnston

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