Communications as a Service (CaaS) is an architecture for communications applications.  CaaS extends the principles of a Software as a Service (SaaS) architecture by adding a control layer based on the Session Initiation Protocol (SIP),  The ability to leverage an application layer signaling protocol in a hosted environment has the potential to radically change the software industry.

The link below is my article on CaaS.  This article begins with a definition of CaaS.  The article then goes on to describe the fundamentals of the SIP protocol and the IP Multimedia Subsystem (IMS), which defines an infrastructure for Fixed/Mobile convergence.  The article concludes with a reference architecture for a CaaS solution.  Subsequent articles will dive deeper into different parts of the architecture.  

 http://msdn2.microsoft.com/en-us/library/bb896003.aspx