There are a few options to make your ring group go straight to voicemail:

1. Inside of your ring group, add a person and under Seconds To Ring* write 0. This will cause the ring group to go straight to voicemail without ringing. 



2. Select the ring group and go to Ring Group Hours > Edit Hours and edit the hours to be 11:30 PM-12 AM on Sundays. Make sure you select the Outside Ring Group Hours action to go to voicemail. This will cause all hours outside of this window to go straight to voicemail.