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

Interview
1

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

Check 🔎

: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.