Dial A for advanced VoIP

Asterisk will run in the background on any modest Windows system.

The software comes with a simple installer, and will detect Capi and Tapi telephony devices connected to your PC.

The latter are analogue, and if you have a full-duplex voice modem, there’s a good chance you’ll be able to use it to work with an analogue phone line, without needing to do any configuration. Capi is a similar interface, used for ISDN connections.

In our test configuration, we plugged the system into two phone lines – a standard BT line via a voice modem, and piggybacking on our small-office phone system via ISDN. We also have an account with Voiptalk, which provides Sip telephony and supports IAX, the Asterisk protocol for linking between exchanges.

Before you install Asterisk, install the Cygwin environment on your PC. Install it into C:\cygwin, then download the Asterisk for Windows installer. Make sure you tell it to use the same directory.

You can start using it out of the box, as the default configuration will let you connect a Sip phone to extension 3000. Start the PBX Manager application, click the Admin menu and supply a blank password. Then you can alter details such as passwords or voicemail settings via a basic graphical interface.

Configuring Asterisk
Now let’s look at tailored configuration using the N95 as a single phone to work with mobile, landline and VoIP calls when we’re at home. Configuration is done via text files, which will be in the asterisk/etc folder beneath the location where you installed Cygwin. The most important is called extensio ns.conf, and it’s here that the ‘dialplan’ is laid out, specifying what happens when different devices send specific sequences of numbers to Asterisk.

It’s divided into ‘contexts’, so an internal phone, such as our N95, might appear by default in the ‘internal’ context, from where you might choose a particular prefix to select the landline, just as you would in an office. An incoming line may be in another context and direct calls to a particular extension. You can also arrange for calls to be transferred, or data passed to programs on the PC, but that’s beyond the scope of this article and you’ll find plenty of details in the PDF book mentioned earlier.

For our setup, there are some extra configuration files: iax.conf controls calls via Asterisk’s own protocol, routed via Voiptalk, so we can benefit from cheap international calls; sip.conf is where we define the details that allow our N95 to connect to the system; capi.conf is for the ISDN line; and tapi.conf is for the analogue voice modems.

Let’s start with a look at sip.conf. It’s pretty simple to define settings that match those in our N95 walkthrough:

[nigel]

type=friend

secret=LetMeIn

qualify=yes

nat=no

host=dynamic

canreinvite=no

context=internal

The ‘secret’ line is the password, and the username we need on the phone is in the square brackets. The context tells Asterisk which part of the dial plan in extensions.conf to use when this phone tries to dial. In our capi.conf file, we tell the system about our ISDN line and that it’s running in MSN mode, where we have several numbers available. In the tapi.conf file – which is what most users will be using – you can modify the settings from the default supplied with Asterisk for Windows and choose how many rings before Asterisk answers.