Call Blocker

01/01/14: Page Origin. 

THE PROBLEM


Starting on the first business day of December 2013 we were deluged by "SPAM" 
telephone calls.  They were persistently trying to find a payday loan deadbeat, 
trying to help us pay off a payday loan (we don't use payday loans), or sell us
all kinds of stuff we didn't want.  We told the bill collector we had never
heard of the person they were looking for, but they kept calling until we gave
them a forwarding number for the the deadbeat, which happened to the the United
States Department of Justice local office, then they quit calling.  Redirecting
persistent bill collectors to the DOJ seems to make them pause and reflect, if
you have already explained you don't know the person they're looking for.  We
have caller ID so when a familiar name or number popped up, after the first 
ring, we just took the phone off hook then replaced it.  We got an average of 4
calls per hour for most days!






THE SOLUTION


Now this very quickly bugged us, so I started thinking how I could stop the
calls or at least automate our response.  I remembered that modems could be made
to go off hook, then hang up, and they squirt out caller ID strings, so I 
Googled Linux call blocking modem software and got a lot of hits.  I settled on
jcblock by Walter S. Heath, it was written in 'C', was GNU licensed, and didn't
require a lot of complex libraries or gui support (this runs on a headless
server).  The software has a whitelist for known friends whose caller ID reads
"Wireless Caller" and a blacklist for known call spammers.  I had a
little problem reading the code (I'm very old and have my own ideas about how
code should be structured), so I reshuffled things, broke multi-page functions 
into single page funcs, compressed vertical space by moving comment lines to 
follow 'C' statements, moved the main() to the top, and added prototypes for all
the funcs.  I removed the tones.c module from the build, because my server
doesn't have a microphone or sound system.  I also added a few of the string 
processing routines from my libUt to facillitate parsing file lines (I inserted 
the code into jcblock.c) and added tracing.  Added a makefile with targets for 
jcblock, truncate, tones, and tar.  The truncate and tones are stand alone with
a main() for testing.  I changed the code so the files weren't kept open and 
allowed a CR NL at the end of the lines in all files so vi would work (I know
emacs is keen, but I still like vi, like I said, I'm old).  I decided to add
some .php code so I could look at the log and manage the black and white lists
via the web.  I also set up the directory and web server so a login was required
to access the Call Blocking pages (to keep the phone number harvesters away).


MODEMS (telco)


Most newer PCs don't have serial interfaces (for an external modem) on the rear
panel (some have a plug on the motherboard requiring a special cable with a rear
metal plate to hold the DB-9).  A lot of the internal modems you find now a days
are win-modems.  Beware of WinModems, they depend of Windoz software to do some 
of the modem's work.  If the vendor's specs only list versions of Windoze, look
for another modem.  We don't have any Windoz running in our house.  
Look here for more on Winmodems USB modems, however, are usually complete stand-alone
modems like external modems and work well with Unix/Linux.  I got a USR model 
5637, USB modem, changed the /dev/xxx in the code and the CID enable string to 
what the USR required.


VOILLA!


Now we have a call blocker.  It answers after the first ring with a short modem
negotiation squall then hangs up, if the phone rings twice, we answer it.  Most
of the bad guy's calling programs hear the modem's negotiation squall and stop
bugging us.  I can put match strings in the blacklist for the name or the number
of the caller, I can even put the area code and exchange without the last 4 
digits so I block all calls from a section of a city (like Washington D.C. or
Austin Tx.), keen!  This area code and exchange idea is especially handy for 
political calls.
Sample output of the callblocker
05/23/2022 14:57:34.553 clnCID: [--RING-- 01]
05/23/2022 14:57:35.429 clnCID: 
		[--DATE=0523--TIME=1457--NMBR=12542446435--NAME=UNAVAILABLE--]
05/23/2022 14:57:35.429 chkBlkLst: blacklist match: UNAVAILABLE -- ------ Cheater
05/23/2022 14:57:40.153 initMdm: Ready For Next Call
05/23/2022 15:52:59.705 clnCID: [--RING-- 01]
05/23/2022 15:53:00.541 clnCID: 
		[--DATE=0523--TIME=1552--NMBR=16822451083--NAME=UNAVAILABLE--]
05/23/2022 15:53:00.541 chkBlkLst: blacklist match: UNAVAILABLE -- ------ Cheater
05/23/2022 15:53:05.265 initMdm: Ready For Next Call

And two test calls: 1st is on the blacklist, 2nd on the white list
05/23/2022 13:07:05.289 clnCID: [--RING-- 01]
05/23/2022 13:07:06.197 clnCID: 
		[--DATE=0523--TIME=1307--NMBR=1xxxxxxxxxx--NAME=WIRELESS CALLER--]
05/23/2022 13:07:06.197 chkBlkLst: blacklist match: xxxxxxxxxx -- ------ FOR TESTING
05/23/2022 13:07:10.921 initMdm: Ready For Next Call
05/23/2022 12:50:25.313 clnCID: [--RING-- 01]
05/23/2022 12:50:26.181 clnCID: 
		[--DATE=0523--TIME=1250--NMBR=1xxxxxxxxxx--NAME=WIRELESS CALLER--]
05/23/2022 12:50:31.273 clnCID: [--RING-- 02]
05/23/2022 12:50:36.225 clnCID: [--RING-- 03]
A Thought
It occurred to me that if the modem does text to speech, it could make an 
announcment or comment to the caller!
Like: "go away!" or something nicer!