[SR-Users] Setting up Kamailio as SIP Proxy

Hi, I am needing some help setting up kamailio as a sip proxy between a provider(trunk) and a test pbx The trunk requires username&password and i would like to add that information to the request within the Kamailio proxy, so that the PBX can authenticate directly by IP (without username&password) through the Kamailio proxy. As a programmer with a limited background in SIP (currently learning), I am keen on understanding the intricacies of this setup at the SIP level. Could you please direct me to resources or documentation that covers scenarios similar to mine? Any help or guidance you could provide would be greatly appreciated. Thanks