Systems and methods for call direction detection on Session Initiation Protocol (SIP) Internet Protocol (IP) Multimedia Subsystem (IMS) include monitoring SIP register messages, such as “401 UNAUTHORIZED” messages inside the IMS core to detect and log the IP addresses of the Proxy-Call Session Control Function (P-CSCF). The systems and methods include monitoring SIP registration messages; determining and storing addresses of P-CSCFs based on the monitoring of the SIP registration messages, wherein P-CSCF addresses are determined from any SIP registration messages having cipher or encryption keys therein; monitoring SIP INVITE messages; and, for a specific call associated with a SIP INVITE message, determining a direction of the specific call based on a comparison of address in the SIP INVITE messages with the stored addresses of the P-CSCFs.