A program to block telemarketing (junk) calls.
(04/2013): Several people have been successful running the
program on a Raspberry Pi processor using a USB modem. The
modems that have been used successfully are: US Robotics
USR805637 and TRENDnet TFM-561U. There is a good writeup on
the details at:
(12/2013): A minor change was made to the truncate.c file to
cause data file record truncation after nine months instead of
after one year. See the 28 December, 2013 entry in the UPDATES
file for details.
(02/2014): Code was added that emits a fax tone to a black-
listed caller. This feature is optional and is designed to
work with fax modems. This option will cause some predictive
dialers to reject the number and remove it from their call
list. See the 20 February, 2014 entry in the UPDATES file for
(02/2014): Added code to add a tag to a call record before
it is written to file callerID.dat. The tag indicates how the
program handled the call. See the 23 February, 2014 entry in
the UPDATES file for details.
(03/2014): Made provision for permanent blacklist records.
These records will not be removed by the routines in the
terminate.c file when they are called. See the 5 March, 2014
entry in the UPDATES file for details.
(04/2014): Changed the method to terminate a call. The new code
is faster, so a blacklisted call is terminated after just one
ring. The telephone touch pad '*-key' method to add a call to
the blacklist may now be used after the second or third ring.
See the 8 April, 2014 entry in the UPDATES file for details.
I have been using the USRobotics model 5686G modem since
October 20, 2010 and have seen no errors. I have therefore
decided to recommend using it instead of the Zoom 3048 modem.
I used the Zoom modem during most of the development work on
this project. However, late in the testing I discovered that
occasionally it did not deliver caller ID data between the first
and second rings. I tried two 3048 units and saw the same result.
I also upgraded the firmware in one of them with no effect. I
sent email to Zoom and got no reply.
If you have a Zoom 3048 modem, definitely use it for evaluation
of the jcblock program. It will demonstrate all functionality of
the program and you can start building your blacklist. The symptom
you will see when the modem fails to deliver caller ID is that a
call that would normally be blocked by an entry in the blacklist
will be passed through. To get error free performance, you will
need to purchase(~$90) a USRobotics 5686G modem.
If you test the program with other modem brands, please report
your results (good or bad) using this site's mailing list or by
contacting me directly from this project's website. I will post
any useful reports I receive. Thank you...WSH
This program connects to a serial port 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.
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 (see the 09/24/10 UPDATES file entry
for more details). 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
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
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
The USRobotics (and probably other modems) must be sent a country
code command if the country of use is other than USA (the default).
The command syntax is: "AT+GCI=XX\r", where XX is the country code.
The codes for the USRobotics modem are as follows (check your modem
'Command Set' documentation for confirmation):
Austria 0A Hungary 51 Norway 82
Belgium 0F Iceland 52 Poland 8A
Cyprus 2D Ireland 57 Portugal 8B
Slovak Federal 2E Israel 58 Spain A0
Denmark 31 Italy 59 Slovakia FB
Estonia F9 Latvia F8 Slovenia FC
Finland 3C Liechtenstein 68 Sweden A5
France 3D Lituania F7 Switzerland A6
Germany 42 Luxembourg 69 Turkey AE
Greece 46 Malta B4 United Kingdom B4
The command should be sent in the init_modem() function after the
modem reset command (ATZ) is sent (the location is indicated in the
listing). Other program changes may be needed to conform to
non-USA country telephone systems.
Real-time prototype system:
The photo shows a prototype jcblock system consisting of a
USRobotics 5686G modem and an eBox-3300 Server-in-a-box (SIB)
purchased from EMAC, Inc. The SIB contains a Vortex86DX
System-on-a-Chip that implements most of the functions of a
standard x86 PC. The system shown draws about 6 watts, so the cost
of running it 24/7 is minimal. The eBox came with a version of
Linux installed that did not have the ALSA audio interface. Since
this application requires a microphone to implement the star (*)
key feature, I chose to boot a stripped down version of Knoppix
from a USB memory stick.
I started with the version included on a DVD with the book:
"KNOPPIX HACKS, 2nd Edition", O'Reilly, ISBN:978-0-596-51493-8
and "remastered" it (trimmed its size) using the procedures
described in hacks #109 and #110 (in addition, I removed all
documentation and Linux source code, etc.). The result was a
version that would fit on a CD, for backup. The procedure results
in a knoppix.iso file which I then used to make USB boot version
using a procedure I found at:
Note that if you don't need the star key feature, the program will
run just fine on the version of Linux that comes with the eBox.
The Knoppix version has the additional advantage that the gcc
compiler, libraries and include files needed to compile the program
are present on the target.
The knoppix system did not contain an Ethernet module that was
compatible with the Vortex86DX chip. The needed module is
r6040.ko. It may be downloaded from a site reached via:
The module depends on module mii.ko, which was included with
The photo shows the Knoppix USB stick installed in a front USB
socket. All application files are contained on the USB stick
installed in the back. This provides non-volatile storage for
them, which is needed for the callerID, blacklist and whitelist
files. The eBox BIOS may be instructed to boot from the USB stick.
During the process of constructing the knoppix.iso file, as
described above, I installed a script called startApp in
/etc/init.d and link S99startApp to it in /etc/rc2.d to provide
for automatic startup of jcblock (the grub loader menu.lst file
instructs grub to boot to level 2). Script startApp mounts the
application USB stick on /mnt and then runs a setPorts script on
it to perform various system administration functions not included
in the knoppix.iso build (some that were forgotten!) before
starting the application program jcblock. As a result, the entire
system starts up and runs when the power is turned on without the
need for manual operations via a terminal and keyboard. Since sshd
and ftpd daemons are started, all maintenance may be performed via
the Ethernet interface. It is especially convenient to maintain
data files using a Firefox plugin called FireFTP. So the system
may be maintained through a GUI from any computer that can run
Firefox (e.g., Windows, Mac, etc.).
There is the issue of interrupting a write to flash memory by a
power failure (or turn-off), which could cause damage to the
memory. The application program is normally blocked in a serial
port read operation, waiting for a call. The only time writes occur
is for a short time immediately after a call is received. So, if
the unit is turned off at other times, no damage will occur to the
USB memory. On the outside chance that memory is damaged, the
application USB memory stick may be easily replaced. Since Knoppix
loads into a RAM disk, no damage can occur to it.
That about summarizes my experience with this project. 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 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 similar to the one I
described above. If your PC does not have a serial port, a PCI
serial port card or USB serial port adapter can be installed at