Anchoring RTP with rtpengine Solved a Different Problem Than SIP Routing
Kamailio could route the signaling perfectly while media still failed. rtpengine made the signaling path and media path explicit instead of treating them as one thing.
SDP
SDP Is Where Signalling Hands the Call to Media
A SIP call can connect perfectly and still carry no useful audio if the negotiated media description is wrong.
SIP Ladder Diagrams Became More Useful When I Added Media Events
A signalling-only ladder can say a call succeeded while the user heard silence; adding SDP and RTP events fixes that blind spot.
SDP Was the Part of SIP Calls I Had Been Ignoring
A SIP call can signal perfectly and still have broken audio because SDP is where the endpoints describe media addresses, ports and codecs.