Using Asterisk in the UK

This guide describes how to get Asterisk working properly in the UK. As each telco's standards differ, services such as caller ID work differently.

Compiling Asterisk

Due to UK caller ID specifications, you will need to make a small modification to the source.
Open up chan_zap.c, and find the line:

Change this line to read

and save the file. Now build asterisk as usual.

On asterisk > 1.2 you can avoid this by adding:

to zapata.conf

BT PRI

BT is the main carrier in the UK. Their PRI lines are, by default, ISDN30e (euroisdn). By default you will have no extra services turned on, so you will need to call up the ISDN management team and get them to turn on this feature (Be aware! It costs extra!).

In my configuration, the PRI is plugged in to span 1 on a TE410P interface. On span 2 i have a Rhino T1 ChannelBank, and spans 3 and 4 are currently unused. My zaptel.conf is below:


Even though only 8 channels are active on the PRI, all channels should be configured in zaptel.conf as shown.

The zapata.conf for the above system is shown below:

Sending Caller ID to the BT PRI

When you placed your order, you may have got a DDI allocation. If this is the case, then you may want the outgoin caller ID set to the DDI of the user making the call. You will first need to contact BT and ask them to add all your DDI numbers to the list of allowed calling id numbers. Once this is done (it takes a few hours) then you can set your outgoing callerid in extensions.conf like so:


By default, BT pass 6 digit extensions in and out of their network.

Incoming Caller ID

Again: By default, BT pass 6 digit extensions in and out of their network.

Therefore in your incoming context, incomingfrompstn in the example here, you will need to specify these 6 digit extension.
For ease of use, i have created a macro to handle dialling a channel and passing off to voicemail:


To execute this for your DDI numbers, an example incomingfrompstn context is shown below:

TDM 400 FXS & BT POTS lines

(SimonHobson, 13 June 2006, mod Dirk Koopman 21 Sept 2006)

Set the card to UK lines spec by adding

to /etc/modprobe.conf (or possibly /etc/modprobe.d/zaptel or /etc/modules.conf depending on distro ?)


BT do not supply Caller ID by default, you can pay extra for this as one of the "Star Services" that are available, or, you can get it free by signing up for "BT Privacy" http://www.bt.com/privacy which signs you up to the Telephone Preference Service and gives you Caller ID for free. To set up for UK Caller ID signalling use (note: I haven't been able to verify this, it came from another forum) :


In UK CID is sent after a polarity reversal and before the first ring. In theory one can set



to zapata.conf. It works for me.

FXO phantom ringing

I had a BIG problem with phantom ringing, after calls ended, there would immediately be an incoming call but you would get dialtone when you answer it. This was solved by adding :

to /etc/zapata.conf

This should not be done for FXS lines as this will prevent CID and answering!

TDM400P & Not Detecting Hangups

Got a TDM400P installed and having problems with Asterisk not detecting hangups? Using BT? If so, contact BT and ask what the "Disconnect Clear Time" setting is for your phone line. Odds are it's probably 100. Increasing it to 800 fixed the issue for me.

"Disconnect Clear Time" is BT's name for CPC.

See http://www.voip-info.org/wiki/index.php?page=Asterisk+Disconnect+Supervision for more info on CPC.