Real life sip is the worst protocol I've seen. There are loads of additions to it that try to fix previous issues. Phone status publishing, nat detection/workarounds, companies coming up with their own extension, etc. etc. Basically forget about things interoperating without proxies that capture and contain/fix all the weirdness. Snom, grandstream and Linksys will not agree on all features natively no matter how you try.
The process for re-invites with proxies in the middle, loose routing, nats, and codec renegotiation is a VoIP engineer's hell.
But all those extensions/rules were created with good intentions.
