Generally in phone settings open the phone icon to dialing pad top right 3 dots than settings it's under either voicemail or call forward option thinking call forward can set to goto voicemail
You can either turn on DND which will automatically send all calls to voicemail while still enabling use of your phone, or you can go to the phone settings/supplementary services and forward all calls to your voicemail number (which you can find in your call settings/voicemail. The advantage of DND over call forwarding is that you can choose to let certain calls come through.
