Yealink SIP-T46G Administrator's Manual page 389

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Step Action
486 Busy Here—Proxy Server
F6
to User A
F7 ACK—User A to Proxy Server
F8 ACK—Proxy Server to User B
Description
The proxy server forwards the 486 Busy
Here response to notify User A that User
B is busy.
User A sends a SIP ACK to the proxy
server. The SIP ACK message indicates
that User A has received the 486 Busy
Here message.
The proxy server forwards the SIP ACK
to User B to indicate that the 486 Busy
Here message has already been
received.
Appendix
377