Although Skype is good at keeping secrets and not documenting architecture some people on the Internet found some evidence that Skype is using SIP for their PSTN connectivity.
If you’ll take a look in Skype config files you may find a section looking like this:
This discovery was commented on a Skype forum. According to some posts on that forum the above configuration may not refer though to the standard SIP protocol: http://forum.skype.com/viewtopic.php?t=26615&sid=dbbc20d59840c2f46b8841e4da82f7a1
Even if Skype client doesn’t connect directly to a SIP server, what is clear is that at a point or other Skype needs to communicate with SIP for PSTN interconnection. “Therefore the Skype gateways for SkypeIn/SkypeOut must translate between Skype Proprietary Protocol and SIP to then interconnect with their various PSTN partners (which require SIP).” – According to a user on the same forum.
A very interesting statement of Niklas Zennstörm, Skype chief executive and co-founder can be read in this article: http://www.vnunet.com/vnunet/news/2126025/skype-prepares-video-internet-service, dated October 2004:
"We may have made the wrong decision. We looked at the whole SIP standard and saw problems with it," said Zennstörm. "We developed proprietary technology to solve these problems."
Now, after more than 1 year, signals of Skype using SIP for PSTN interconnection started to appear all over the Internet.