When the caller hangs up first, which direction is the BYE sent?

Interview
1

Q. When the caller hangs up first, which direction is the BYE sent?

a) S-CSCF → HSS
b) P-CSCF → PCRF
c) Callee → Caller
d) Caller → Callee

Check 🔎

:arrow_right: Correct answer: Letter d.

In SIP/IMS, when the caller hangs up first, a BYE message is sent by the caller to terminate the session. This BYE is routed through the IMS entities toward the callee.

  • The BYE message is part of the SIP signaling used to terminate established sessions.
  • It follows the same SIP path as the initial INVITE, in reverse.
  • No need to contact the HSS or PCRF directly for this action.

Q. When the caller hangs up first, which direction is the BYE sent?

a) :x: S-CSCF → HSS
→ Incorrect. The HSS is not involved in SIP BYE signaling—it handles user authentication and profile queries.

b) :x: P-CSCF → PCRF
→ Incorrect. PCRF manages policy/QoS, not SIP signaling.

c) :x: Callee → Caller
→ Incorrect. If the callee hangs up first, then this would be correct—but in this case, it’s the caller.

d) :white_check_mark: Caller → Callee
→ Correct. The BYE is initiated by the caller and sent toward the callee to terminate the SIP session. The party terminating the call sends BYE to the peer.