There are no time-sensitive settings on an OBi.
There are no settings to do what you what in the normal settings at
voice.google.com.
However there are settings available through the Legacy pages. At
voice.google.com click the three horizontal lines in top left corner. Click 'Legacy Google Voice'. At top right, click the cogwheel and in the drop down menu click 'Settings'. If necessary select the Phones tab. Click the relevant Edit button. Click 'Show Advanced Settings'. Under 'Weekdays' click 'Use custom schedule'. Configure your desired times. Click on 'Save'. Repeat for Weekends.