It has taken Cisco a while to finally get around to making CallManager SIP compliant. First they had to by Dynamicsoft (a former client actually back in 2001) and then they had to integrate it all into their existing code base.
What's ironic is their subsidiary, Linksys, was SIP compliant from the time they bought SIPURA, and got a bottom up strategy going, rather than a top down. The SIPURA acquisition still is being woven into the Linksys/Cisco food chain in many ways. The Linksys IP PBX called the 9000 provides a lot of really cool CallManager like features for fractions of the price of what a Call Manager can deliver.