Program: jcblock
A program to block telemarketing (junk) calls.
PROGRAM DESCRIPTION:
This program connects to a modem and listens for the caller ID
string that is sent between the first and second rings. It
records the string in file callerID.dat. It then reads strings
from file whitelist.dat and scans them against the caller ID
string for a match. If it finds a match it accepts the call. If
a match is not found, it reads strings from file blacklist.dat
and scans them against the caller ID string for a match. If it
finds a match to a string in the blacklist, it terminates the junk
call.
The program also updates the date field of a matching whitelist or
blacklist entry. Entries that are old may then be identified so
that they may be removed. Note that the program will operate with
only a blacklist.dat file defined. A whitelist is not required.
Functions to manage the truncation (removal) of records from the
blacklist.dat and callerID.dat files are present in file
truncate.c. Records in the blacklist.dat file that have not been
used to terminate a call within the last nine months are removed.
Records in the callerID.dat file that are older than nine months
are removed. The operations are performed every thirty days.
Alternatively, entries in the .dat files may be edited manually.
EDITORIAL NOTE: The next few paragraphs describe a method for
detecting a star (*, asterick) key press using a mic to listen
for the tones produced by a modem's speaker. The performance of
this method has always been marginal. An ATian modem provides a
better way. It is a voice/FAX modem. As such it contains circuitry
to detect touchtone keypad entries. This feature is now used to
detect a *-key press. Search file README2 for 'ATian' for more
information.
An additional feature is supported by functions in file tones.c.
The program will add a record to the blacklist.dat file for the
current call if the operator presses the star (*, asterick) key on
a touch tone telephone handset during an allotted time period. The
program detects the tone via a microphone placed near the modem
speaker. Functions in file tones.c detect the presence of tones
(941 Hz and 1209 Hz) produced by pressing the star (*) key. If an
answering machine is installed on the same line this feature must
be restricted to be active only if a call is answered after the
second or third ring (the answering machine *must* also be set to
answer on the fourth or later ring). If an answering machine is not
installed on the line the star key feature is available for calls
answered after the second or later ring. This option is controlled
by a conditional compile flag. Also see the NOTE: in the file
concerning an ALSA version conditional compile selection.
The program requires a serial modem that can deliver caller
ID and contains a speaker. The recommended unit is the USRobotics
model 5686G. It will return caller ID if it is sent command:
AT+VCID=1. Note that the modem is used just to detect a call's
caller ID and the star key tones; the modem's normal communication
function is not used. The program may be terminated by sending
it a SIGINT (Ctrl-C) signal or a SIGKILL signal.
The program runs on a standard PC (it was written and tested
on a Dell Dimension B110 running Ubuntu). The features
supported by functions in files truncate.c and tones.c may be
removed by disabling conditional compile flags DO_TRUNCATE and
DO_TONES, respectively, in file jcblock.c. The default serial port
specifier is: /dev/ttyS0. The program will accept an option to
select a different port. The syntax of the call is:
jcblock -p /dev/portID
The entire program may be compiled with the following command:
gcc -o jcblock jcblock.c tones.c truncate.c -lasound -ldl -lm
Linux installations may or may not install the libasound library.
It is usually installed in /usr/lib. Also, the tones.c file
includes file asoundlib.h, which is usually located in directory
/usr/include/alsa. If not installed, they can be by using
the distribution's package manager. For example, for an Ubuntu
distribution, the Synaptic Package Manager may be used to
install alsa-base, to install the library, and alsa-source, to
install the source files. If /usr/include/alsa is not yet present
install libasound2-dev. It is also a good idea to install
alsa-utils in case testing/debugging of the sound interface is
needed. Here is a full list of the alsa packages I have installed
on my Ubuntu 10.04 machine (you can show the ones installed on
your machine by entering 'alsa' in the Synaptic Package Manager's
Quick Search: window):
alsa-utils alsa-base alsa-source
libasound2 libasound2-doc libasound2-dev
libasound2-plugins libsdl1.2debian-alsa linux-sound-base
gstreamer0.10-alsa bluez-alsa libao2
No doubt some of these are not needed for the jcblock program,
but if you install all of them, jcblock should run.
If jcblock is run on a desktop or laptop, a microphone must be
placed near the modem's speaker and turned on. The volume level
must then be adjusted so that when the star key is pressed, the
audio generated causes the program's tone filter thresholds to be
exceeded. Mic audio volume may usually be set by a Preferences
option. For Ubuntu the path is: System|Preferences|Sound. Then make
sure the proper sound device is selected under Hardware. The mic
volume may then be set under Input. The setting depends on the
position of the mic relative to the modem's speaker. The slider is
quite sensitive. Another way is to use the alsamixer program:
choose the sound interface (F6) and the Mic slider (arrow keys).
Using this program you have a numeric setting displayed that can be
reused. Remember to keep the phone you answer with away from the
mic, since the mic will respond to the tones received by the
handset!
For continuous use, the program should be run on a low-power single
board computer so that it can be left on all the time.
More on the whitelist:
As a result of operational experience it became evident that
the addition of a whitelist capability would be helpful. The
whitelist file (whitelist.dat) holds entries containing the
telephone numbers of calls the user definitely wants to accept. The
program checks for caller ID matches of these entries first. If a
match is found, the blacklist check is bypassed and the call is
accepted.
The issue came up when it was discovered that some telemarketers
use cell phones! For cell phone calls, the 'NAME =' field in the
caller ID string is usually: 'Cell Phone XX', where XX is the
originating state designator (e.g., CA for California). So using
the 'NAME =' field as the search string in a blacklist entry would
cause all cell phone calls from the indicated state to be blocked!
In addition, the telemarketers use several cell phones (probably
as employees come and go), so using the 'NMBR =' field in the
caller ID as the search string does not catch new cell phone
callers. Adding a whitelist partially solves this problem.
With a whitelist, the user can include entries that contain the
cell phone numbers of calls they definitely want to receive from a
specific state (e.g., CA). Then the cell phone 'NAME =' field for
the state (e.g., 'Cell Phone CA') can be used as the search
string in a blacklist entry. With this arrangement all cell phone
calls from the state will be blocked *except* the ones with
numbers present in the whitelist.
As another example, a person might want to receive calls from just
certain numbers and no others (an elderly person, for example). In
that case the records for the numbers of interest would be placed
in the whitelist and a single record placed in the blacklist that
contains a field that is present in all caller IDs (e.g., 'NMBR').
Then all calls not matched by records in the whitelist would be
terminated.
Wrap-up:
The most detailed source of documentation for the project it the
comments in the listings. I have added many for this purpose.
The system runs continuously at our home and terminates junk calls
indicated in the blacklist file. After about six months the number
of junk calls we received decreased to only a few a week as the
telemarketers removed us from their call lists.
The easiest way to get started with this project is to just compile
the program and run it on a standard PC. It works fine that way,
but you have to leave your PC on all the time for it to be useful in
terminating calls. If you do that anyway then you are all set!
Otherwise you should eventually consider installing it on a
dedicated low power embedded computer system.