To sum it all up, IP phones conforming to SIP effectively allow IP calls to be automatically routed to compliant handsets.
The technology is able to determine the end system that will be used for communication sessions and automatically sets the appropriate parameters at both ends of communication. It then manages both call transfer and call termination. SIP also allows users to initiate and receive communication from any location.
But the hindrances are the firewall issues, unnecessary complexity, and the cost associated with adopting SIP. Skype technology has an edge here; the biggest plus point for Skype being that there are more Skype users today than SIP.
Today VoIP technology has reached a state where we have the best hardware and the best services. The research behind it and the way the protocol will move forward in future will only make telephony cheaper, that is, unless the big VoIP business tycoons make sure you still pay a premium. With Skype's acquisition by eBay (somewhere between $3.7 billion to $4.1 billion) free Internet phone calls may not be free anymore! The question that looms today is "Will eBay seek to recoup its huge investment by charging for the service?"