I have steps below on how to turn it on and since your concern is to turn it off please do the vice versa. Hope it helps.
Forward calls to your voicemail or another phone number
Your phone is ringing, but you can't answer? Let the call go to your
voicemail, or forward it to another number. Call forwarding is a
1. Select settings > calls > call forwarding.
2. Select when to forward the incoming voice calls:
all voice calls - Forward all calls.
if busy - Forward when busy.
if not answered - Forward when not answered.
if out of reach - Forward when the phone is switched off or out of
network coverage for a certain length of time.
if not available - Forward when busy, when not answered, when the
phone is switched off, or when out of network coverage.
3. Select activate > to voice mailbox or to other number.
4. If if not answered or if not available is selected, set the length of
time after which a call is forwarded
Nokia 311 User Manual Page 30 Forward Calls To Your Voicemail Or...