Drgeoff's point is: there is no setting on your OBiTALK device that would control this behavior. It is entirely up to your service provider to send the call to your device (which is a SIP VoIP client registered on their SIP server), and to ring it for some defined time interval, and, if no answer, to then give up and forward the call to their voicemail server.
You need to work this out with your service provider. It might be as simple as a call routing or call treatment setting on their web portal for your account.