If the callee is busy, what SIP code is returned?

Q. If the callee is busy, what SIP code is returned?

a) 603 Decline
b) 486 Busy Here
c) 202 Accepted
d) 404 Not Found


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:arrow_right: Correct answer: Letter b.

In a VoLTE call, SIP signaling handles session setup via the IP Multimedia Subsystem (IMS). When the callee (called UE) is busy (already in a call and can’t accept another one), the terminating UE or its P-CSCF/S-CSCF will respond with the SIP code 486 Busy Here.

This code is a 4xx class response, meaning it’s a client-side rejection — the UE is reachable, but temporarily unable to complete the call due to being engaged.

Q. If the callee is busy, what SIP code is returned?

a) :x: 603 Decline
→ Incorrect. 603 Decline is a global rejection, often used when the user intentionally refuses the call — not necessarily because they’re busy.

b) :white_check_mark: 486 Busy Here
→ Correct. This is the standard SIP response when the callee is busy, and the rejection comes from the UE or its IMS node.

c) :x: 202 Accepted
→ Incorrect. Indicates that the request has been accepted for processing — not related to busy conditions.

d) :x: 404 Not Found
→ Incorrect. Returned when the callee doesn’t exist or the IMS cannot resolve the target URI.