Function contact 

pub fn contact(base: &str, instance: &InstanceId, reg_id: RegId) -> String

The Contact a REGISTER for one flow carries.

base is the contact as it would be without Outbound — <sip:alice@192.0.2.5:5060>. The two parameters are appended as header parameters, after the angle brackets, because that is where §4.1’s and §4.2’s grammars put them: +sip.instance is a contact-param and reg-id is too. Putting either inside the brackets makes it a URI parameter the registrar will not look at.