Request-URI

Also known as Request-Line-URI or R-URI. The first line of the request message indicates the destination for routing the message.

If the message is routed through multiple hops, this header changes each time to indicate the next destination. Similarly, if the call is redirected, the header is changed to show the new destination.

Via

Every intermediate proxy that has been involved in the routing of the request is included as a SIP URI. These are used in reverse order to process response messages.

To

This header indicates the original intended call target. It contains a SIP URI and can include a display name.

• On outgoing calls, the identity of the called party is not known at the time of the initial INVITE. Therefore the To: field only contains the information necessary to route the call. That is, the dialed digits after any short code and prefix manipulation.

From

This header indicates the identity of the caller. It contains a SIP URI and can include a display name.

Contact

This header indicates the return address for responses to the call request. This is a SIP URI.

The SIP URI is similar to the From header. However, if anonymous calling is enabled, the field becomes semi-anonymous. For example; Contact: <sip:anonymous@135.55.86.70:5060;transport=udp>

P-Asserted-Identity

The SIP URI is similar to the From header. The field is unchanged even if anonymous calling is enabled.