A VoLTE call involves both SIP signaling (through IMS) and bearer setup (through EPC). When you dial a number, your phone sends a SIP INVITE through the P-CSCF to the S-CSCF. The S-CSCF routes the call to the terminating side. Meanwhile, the PCRF instructs the PGW to create dedicated bearers (QCI 1) on both the originating and terminating sides for the voice media.