Imagine an IP voice and unified communications system that can be integrated into any application and customised to meet business needs. Sounds great, right? Well that project is the Asterisk IP-PBX and it's free to use and you get the source code. A far cry from proprietary PBX systems perhaps, but Asterisk has a vibrant ecosystem and is replacing systems from more established telephony vendors.

Following interviews with the leaders of the Horde and Free Telephony projects, the Open Source Identity series talked to Asterisk founder and Digium CTO Mark Spencer about how one application can have such a profound effect on businesses and how open source can be a tough competitive landscape.

When did you first discover free software and begin to develop code using open source development tools?

I discovered Linux and Unix around 1993 while still attending Auburn High School. In 1994, I attended the Research Science Institute at MIT and got to know and love the Athena system. When I came home, I wanted to use it and learned Linux. Later that year, I got a job at ViperNet (which is now part of Deltacom), as its chief systems administrator at the ripe old age of 17, barely old enough to drive myself to work.

The birth of Asterisk is quite a famous story — you needed an inexpensive PBX to run your Linux support business, but what inspired you release it as an open source project?

I had success with Gaim as my first major open source project. As a result, it just seemed natural to release Asterisk as open source as well, although it would be a few years later before I realised just how significant that would be.

Now Asterisk is a huge success story with millions of people all around the world using it for telephone calls every day. In addition to being zero cost, what do you think has made it such a success?

Asterisk has been successful in large part due to the nature of the industry — it's a broad product with lots of highly technical contributors, it has a lot of demand for customisation, the high price of proprietary alternatives and opportunities for lots of companies to make money with an open source alternative. Furthermore, there really wasn't a single gorilla that was a monopoly in the industry, so it was much easier to break in as a new entrant. Not to mention on top of that, Linux really had paved the way to where open source was something people already had some understanding of, if not specifically in the telecom or PBX sectors.

On the technology side, I think Asterisk's success was greatly built upon the strength of the contributions. It's pragmatic support of both new VoIP technologies and traditional telephony and wide host of applications that worked right out of the box. We didn't try to pick the technology that would win, we just picked a framework to support all the technologies anyone cared about and let the markets decide the winners — and there are different winners in different markets.