All posts for the month October, 2009

Humphrey Littleton, before he died, used to host a comedy programme on Radio 4 in the UK called “I’m Sorry, I Haven’t a Clue“.  After his death, the BBC put up several extracts from the show on their site, one of which just makes me laugh and laugh.  The programme often featured audience feedback from a Mrs. Trellis, of North Wales, as in this case, the Mornington Crescent Audience Response Survey.  I’ve seen several transcriptions on the web, but none are quite right, so here’s my own:

“We asked 20,000 listeners ‘How would you rate your level of Mornington Crescent satisfaction? Excellent, good, or merely well above average?’  and the reply we got back, …,  came from a Mrs. Trellis of North Wales, who I see has ticked the box marked ‘Neither Good nor Bad,’ and also the boxes marked ‘Poor,’ ‘Very Poor,’ ‘Really Extremely Poor,’ ‘Words Begin to Escape Me as to Quite How Poor,’ and ‘Buttock-clenchingly Piss-Poor’.  In fact, Mrs. Trellis sent us back the wrong form, and if there’s anyone listening at Virgin Rail wondering where it got to, …”

Asterisk is an open-source PABX.  It can connect existing telephones, “soft phones” running on a PC, and physical IP phones to the VOIP networks and to existing “POTS” telephone lines.  Connectivity to existing telephones and exchange lines is provided by either external hardware boxes such as Astribank (for large systems) or PCI plug-in cards such as Digium’s TDM400P.  Cards are also available to connect to digital lines such as T1/E1 to provide multiple lines.

Using Asterisk, I can connect telephones in my children’s bedrooms to the one in the kitchen, and allow them to make phone calls (with restrictions).  I can also connect to my home telephone system from the Internet and call my wife on a real phone from my iPod Touch, or make a call apparently from home when I am in a conference room in the USA.  Also, I can provide new telephone numbers in minutes (via VOIPuser for example) which connect directly to a particular phone in my house, or my PC wherever it is in the world that day, or to an automated message.  I can receive voicemails and have them sent to me by email.  I can also write simple teleohony apps which (for example) tell my wife how many emails she has when she dials *90 on the phone in the kitchen.  The telecoms world becomes my oyster.

Asterisk is available in many forms, both in source and binary distributions designed to run on a PC.  The easiest approach is to dedicate a PC to running Asterisk (though mine also runs a firewall for my home network and a disk server).  Pre-packaged distribtions for this are available and the one I prefer is Trixbox.

Getting Asterisk to work in the UK with caller-id can be tricky.  Having struggled with it several times and found a solution which works, I thought I’d write it down both for my own reference and to help other people struggling with this problem.

  1. Install Trixbox from CD.  Trixbox is based on the CentOS Linux distribution.  Mind, it will find and destroy *all* your hard disks at installation time.
  2. Upgrade to Asterisk (I think)
  3. # yum clean all
    # rpm -del –nodeps kmod-dahdi-linux dahdi-linux
    # yum -y update           (this takes a while)
    # yum install kmod-dahdi-linux dahdi-linux

  4. Then edit /etc/modprobe.d/dahdi to add a line as follows:
  5. options wctdm opermode=UK fwringdetect=1 battthresh=4

  6. Then edit /etc/asterisk/chan_dahdi.conf to add the following lines after “usecallerid=yes”
  7. cidsignalling=v23

NOTE that Asterisk seems to refuse to use the outgoing line after a reboot until the line is unplugged and plugged back in again!