A VoLTE call begins when the originating UE sends a SIP INVITE message through the P-CSCF to the S-CSCF. The INVITE carries an SDP offer describing the codecs the UE supports, typically AMR-WB for HD voice. The S-CSCF evaluates its routing logic and forwards the INVITE toward the terminating network. The terminating S-CSCF delivers the INVITE through its P-CSCF to the called UE. Meanwhile, a 100 Trying response propagates back to the originator confirming the request is being processed.