What's the application for "Forward" of Router Setup / VoIP Call in?

Define each item on Forwarding below.

Name: Specify a profile name. Please use UNDERLINE to instead of SPACE due to the HTTP protocol limitation.

Always: Always redirect forward to this IP(or URL) / Phone number, all incoming call will be forwarded to the assigned IP here.

On Busy: Redirect forward to this IP(or URL)/Phone number when busy, all incoming VoIP call will be forwarded to the assigned IP here when this line is busy.

No Answer:Redirect forward to this IP(or URL)/Phone number when no answer over the time No Answer Sec, an incoming VoIP call will be forwarded to the assigned IP here when ring time over the defined No Answer Sec.
Below is the Configuration method for users want to forward the call to the IP of 172.16.39.2 for calling to 66390001 directly, please also revise the call number to 63986399.

Configuration on VoIP Call in below:

Forwarding: