[Serusers] Features with SER/Asterisk

My questions involve using SER/Asterisk in a carrier style setup and not an office PBX. I have SER working with Asterisk. Asterisk has the ability to do standard call features as described at http://www.voip-info.org/wiki-PBX+features My question is how does someone with a SER/Asterisk setup get SER to use those features? Explanation below. For example, say a SIP caller forwards his calls to a PSTN number like a cell phone. If this person is called by another SIP phone, the Asterisk server is never contacted, correct? How does SER know to forward the call? Does that make sense? Some of the documentation I've read says that many features are handled by the SIP phone itself. I can understand that when it comes to caller-id but what about call forwarding. Surely I'm not expected to have my internet connection and SIP phone online just to ensure my calls are forwarded. Are most carrier installations setup using MGCP? I understand that's how AT&T is setup and that's why they can't be used from behind a NAT. Bill