Call direction detection on SIP IMS

A method comprising:

• monitoring Session Initiation Protocol (SIP) registration messages;
• determining and storing addresses of Proxy-Call Session Control Functions (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.

A processing device comprising:

• one or more processors;
• and
• memory storing instructions that, when executed, cause the one or more processors to
• obtain monitored Session Initiation Protocol (SIP) registration messages;
• determine and store addresses of Proxy-Call Session Control Functions (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;
• obtain monitored SIP INVITE messages;
• and
• for a specific call associated with a SIP INVITE message, determine 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.