SIP 486 Busy Here vs 600 Busy Everywhere
SIP 486 and 600 both indicate the callee is busy, but they differ in scope. A 486 means this specific endpoint is busy (the user may be reachable on another device), while a 600 means the user is busy on all devices and cannot be reached anywhere.
Description
The callee's end system was contacted successfully but the callee is currently not willing or able to take additional calls at this end system.
When You See It
When the callee is already on a call and does not have call waiting. The user explicitly rejected the call, or all lines are in use.
How to Fix
Wait and try again later. The callee may become available when their current call ends. Consider leaving a voicemail if available.
Description
The callee's end system was contacted successfully but the callee is busy and does not wish to take the call at this time. Unlike 486 (Busy Here), this is a definitive response.
When You See It
When the callee has rejected the call on all devices. No other endpoints should be tried. The user is universally busy.
How to Fix
Wait and try again later. No alternative contact points exist. The callee must become available on their own.
Key Differences
486 is endpoint-specific — the user might answer on a different registered device (desk phone, mobile, softphone).
600 is a global failure — the user is busy on all endpoints and no device can accept the call.
486 is a 4xx (client failure) response; 600 is a 6xx (global failure) response.
A SIP proxy receiving 486 from one endpoint may fork the INVITE to other registered contacts.
A SIP proxy receiving 600 should not try other endpoints — the user is definitively unavailable.
When to Use Which
Return 486 when a specific UA (phone) is busy but the user might have other registered endpoints. Return 600 when the user is known to be busy across all devices — for example, their presence status indicates they are in a meeting. Proxies should use 600 to stop forking attempts.