The graph below is an attempt of the simple state diagram that rules message
delivery.
It's split in a half according to whether you are inviting or answering; and
on the sides are the different sbf's state.
The middle zone (for both types) is the message type with an arrow indicatiing
the destination (left is us, right is the server); except for the only two
two-letter codes right in the middle, which show the common state to both
sides.
Once the 'es' state is reached, the difference is forgotten (even tho it's
kept, it's never used) and both types of sbf are handled the same way.
------------------------+------------------------
invite | answer
------------------------+------------------------
state | | | state
------------------------+------------------------
----+ xfr->
xf | |
----+ <-xfr <-rng +----
cp | \ / | cp
| \ / |
----+ cp +----
| / \ |
re | / \ | re
| / \ |
----+ usr-> ans-> +----
| | | |
us | <-usr <-iro | an
| | | |
----+ cal-> <-ans +----
ca | | | |
----+ <-cal / |
jo | | / | es
----+ <-joi / |
es | \ / |
V es V