Comcast now has 6.47 million subscribers to its VoIP (Voice over Internet Protocol) service, available in 39 states, giving the cable operator more residential customers than Qwest Communications International, according to Comcast.

To people who have followed the development and implementation of VoIP, the revelation may be surprising. VoIP started out as a cutting-edge technology used by startups hoping to compete with the established telecommunications carriers. But at least initially, it wasn't the kind of technology that old-world operators or cable companies would employ.

Today, in addition to the cable operators, even the traditional phone companies offer VoIP services. And many of the original VoIP providers are struggling or have failed, sometimes ingloriously. Take SunRocket. In 2007, it abruptly went belly-up practically overnight without notifying any of its 200,000 customers.

Vonage, perhaps the most recognizable name in VoIP, is still around but has suffered under the weight of a series of intellectual property lawsuits filed by Nortel, Verizon, Sprint and AT&T.

But while the companies offering the service have changed, the number of subscribers around the world has grown dramatically. Comcast only began offering VoIP in 2005. The International Telecommunication Union expects 250 million VoIP users worldwide by the end of 2011, excluding computer-to-computer systems such as Skype.