Re: Called Party Information on INVITES

On Mon, 2012-07-16 at 12:32 -0500, Pena, Christian wrote:
> I have a SIP Trunk with a bunch of DIDs under it. When a call comes into
> the SIP Trunk, the CALLED parties number shows up in the To header. The
> end user is looking for the called party number to show up on the
> Contact header instead. However on my INVITES, that shows the SIP trunk
> username(which is one of the customers phone numbers).

The *requirements* are:

1. The Contact of the INVITE is a URI to which later requests in the
dialog can be sent by the called party.  The called party shouldn't
depend on this URI having any particular format.

2. The request-URI (in the first line of the message) is the URI toward
which the request should be forwarded.  (Of course, various forwarding,
contact resolution, and redirection operations will change the
request-URI in downstream versions of the INVITE.)

By *convention*:

3. The To URI gives the "identity" of the caller.

4. The From URI gives the intended callee, which usually is the
request-URI which the UAC put into the first INVITE.

In reality, the To and From URIs are used only as documentation, in that
no automated action is taken based on their contents (although the UAC
must be careful to ensure that they are syntactically correct, as
downstream elements must be able to extract the "tag" header
parameters).