You are a little vague, perhaps you can clarify.
If you are referring to SIP (Session Initiation Protocol), as in for VoIP (Voice Over IP), I don't know of any native support ever being included on a Cell phone. That is usually reserved for the VoIP apps to bring their own, like Google Voice and Vonage do.
It's not BLOCKED at the phone level, if that's what you mean. There are several VoIP apps that work. However, the carriers sometimes block certain VoIP apps from running on their network.