Skype had started a closed Beta of its SIP program for businesses to use on their PBXs or unified communications systems. This week, the Beta is open for anyone who wants to try it. To use this service, a person needs to have a corporate type Skype registration and a business SkypeIn number. This kind of SIP Trunking offered by Skype connects the PBX or any UC system of a company to Skype, thus allowing pure Skype, SkypeIn and SkypeOut calls through the system.
Note that Skype for SIP does not allow you to port a number to it - you will have to use your business SkypeIn number, or buy one if you don't have, at $6.95. This is quite discouraging for companies that carry some goodwill in their existing numbers. Outbound calls have a rate of 2.1 cents per minute for 36+ destinations. Also, there is no 911 support. Read more on Skype for SIP. If you want to try Skype for SIP, go get it there.