Adjusting the length of time a call will ring before being routed to voicemail is only possible on GSM LTE.
From your cell phone, dial **61*18056377243**seconds# where seconds is the length of time the call will ring. Acceptable inputs are 5,10,15,20,25 or 30 seconds. So if you wish to let the phone ring for 20 seconds before going to VM, you'd dial **61*18056377243**20#.
It is not possible to change the ring time on a Warp 5G line.