Voice over IP (VoIP)

Open Source IT uses Asterisk for its own telecommunications needs.

This enables us, for example, to answer calls to our UK office from Germany, when we're busy in the UK office.

Asterisk also enables us to both make and receive calls through our standard contact numbers, from any remote location with an Internet connection.

See below for details of how we've set this up, and ideas on how we could do the same for you.

Open Source IT has two offices, one in the UK, and one in Germany. Each office has standard telephone lines (analogue in the UK, ISDN in Germany), with an Asterisk server containing the appropriate interface card.

The Asterisk servers are connected to each other using a VPN over the Internet (using a VPN isn't essential; we just happen to have it in place anyway, so it's a convenient way to let the servers talk to each other).

When a call comes in (for example, to the UK office), the UK Asterisk server catches the call, rings the UK-based telephones, but also passes the call (by IAX - the Inter-Asterisk eXchange protocol) to the Asterisk server at our German office, which then rings the phones in the German office. One of the nice things about Asterisk and SIP phones is that we can manipulate both the name and number which are displayed on the ringing telephones, so that it's clear whether the caller has dialled the UK or the German office number.

The system also gives us the facility to make outgoing calls from the other office, so that calling a UK number from our German office does not involve an international phone call (cheaper for us, and more convenient for whoever we called to be able to call the displayed number back again), and vice versa.

We also have Asterisk installed on the server which the DNS entry for "open.source.it" points to, and this enables us to accept direct SIP calls to addresses such as "821471@open.source.it", which can be dialled on many SIP telephones.

Finally, we also use SIP phones (both cabled and wireless) so we can have an "office phone extension" with us wherever we are.

The system is not quite as complex or sophisticated as some of the solutions we've put in place for our bigger customers, but it meets our needs, and gives us complete control over how our telephone system works.