SIP
486 Busy Here
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 This Code
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.