Forward on no answer redirect calls that have alerted your phone but have not been answered for some reason.

By default this function is only applied to incoming external calls to you. However if required internal calls can also be selected.
When enabled, all calls matching the settings are forwarded to the number set as the forward on busy/no answer destination. That number can be internal or external.
No answer is defined as having been presented to your phone for your no answer time (default 15 seconds).
Hunt group calls are not forwarded.
The 'forward on busy' and 'forward on no answer' options use the same forwarding destination number. If no number is set, they use the same destination as set for 'forward unconditional.'
If you also have voicemail enabled, if the the forwarded call is still unanswered after having rung for your no answer time (default 15 seconds), the system will redirect the call to voicemail. Note that this is not always possible for calls that have been forwarded to an external number.