When it happens
- The callee is talking and has no call waiting.
- A device with a limit of one call per line.
- Some carriers answer 486 when all their channels to that destination are busy.
What to check
- Nothing to fix if the person really is busy.
- If it happens on every call, check the per-line call limit and call waiting on the device or extension.
What the response looks like
SIP/2.0 486 Busy Here
Via: SIP/2.0/UDP 203.0.113.10:5060;branch=z9hG4bK776asdhds
From: <sip:101@pbx.example>;tag=1928301774
To: <sip:+15551234567@pbx.example>;tag=a6c85cf
Call-ID: a84b4c76e66710@203.0.113.10
CSeq: 2 INVITE
Reason: Q.850;cause=17;text="User busy"
Content-Length: 0