I agree with Eric.
More on the "it doesn't matter who is the caller" stuff:
On the RFC 4474 side, RFC 4474 allows for the sender of the request
to provide the identity assertion. Yes, indeed, this is the calling
party.
There is no "response" identity. However, the callee may send its own
request in
the reverse direction to provide it's own identity, as per
draft-ietf-sip-connected-identity-05.txt.
In other words, in SIP also there is no implied "direction" to this,
even if
technically, the origin of the protocol used a client/server model (as
in,
request/response).
I think we'll have to write up a "high level" description on how these
pieces
fit together.