Hi,
So you need first of all to make a forwarding rule for the particular extension so when a call comes in and there is no answer or the line is busy, you forward the call to Voice mail.
Example -
Go on the main extension (e.g. 100) and edit the Extension.
Go to Forwarding rules > Advanced.
Rule Type - No Answer
Hours - All /office /out of office...
Call Type - All calls
Action - Forward to Voicemail with the Ext number
Add the rule
Apply / Ok.
So in scenario when a call comes in and there is no answer or busy it goes to Voicemail.
If you need any assistance, let me know.
Thanks