How it works
An endpoint may return 486 in response to an INVITE when it is occupied. Other contacts or alternative routes may still be available. The PBX can apply a preconfigured fallback, such as a queue, another agent or voicemail.
Business example
An agent is already speaking to a customer, so the next incoming call is routed to a colleague rather than being dropped.
What to consider
Distinguish busy status from rejection, failed registration and network faults. The actual fallback depends on the call-routing policy.