Session Initiated Protocol is the next universal step toward comprehensive IP telephony.
What SIP provides essentially is the option to eliminate the multiple analogue and digital lines that a business utilises and replace them with a single online SIP trunk service. Providing considerable cost savings as well as ability to route calls off the traditional telephone system being used. Providing cost savings especially if businesses operate across different sites as these calls are often provided free.
What SIP also provides is extremely low transition time to install and this also has immense implications to disaster recovery. All this can be achieved on your existing IP telephone system.