The current API for call_handling does not allow for modification of routing options. The GET returns any routing settings returned, but PATCH does not let you modify them.
This is a huge limitation since the call_handling API is used in both call queues and auto receptionists. Since we cannot modify routing, auto receptionists cannot be setup completely through API.
If you include forward_to_id this will set the auto receptionist to forward to the voicemail of whatever extension ID you provided.