Marcelo Rodriguez rounded up five products that connect Skype and SIP products in his post, Is a Skype-SIP Peace At Hand? 

• VoSky Exchange
• Skype2PBX
• Uplink
• ChanSkype 
• Pika Connect for Skype

We all want interop, and these products are gaining loyal followings. They build audio pipes between SIP and Skype voice callers. We've been calling these Level Three Skype integration in our Skype Journal Connectivity Maturity Model. 

Skype Journal Connectivity Maturity Model

Level 0. No connection.
What's VoIP? What's Skype?

Level 1. Skype indifferent.
Devices doing nothing but input or output like the most basic of USB phones. On the software side, the only software is Skype.

Level 2. Skype aware.
Configurations are Skype-aware or Skype-smart devices, like the Kensington Vo300, the YapperNut YapperBox.

Level 3. Skype conversant.
Level 2, plus audio pipes between apps, especially across the SIP barrier. You call with your SIP phone, something happens in between, and my Skype phone answers.

The move from Skype to SIP at Level 3 costs you all the benefits of rich conversation. You lose:

• Availability and geopresence
• Mood messages
• Caller authentication
• Access to caller profiles
• Launching text chat or video in the same call
• File transfer and folder sharing
• Voice messaging
• Access to Skype voicemail
• Skype multichat and conferencing
• Broadband audio quality 
• End-to-end encryption
• Chat/call permalinks 
  (e.g. skype:?chat&id=%23leedryburgh%2F%24evanwolf%3Bd5b446f89da627a3)

Level 4. Skype equivalent.
Level 3, plus restoring most of the missing elements. 

Does this model work for you? What's Level 5? What do you call it when the other system has capabilities beyond or different from Skype and you can't translate them?