not sure how you set this up on your end but on the forwarding tab if you have unconventional turned on and to voicemail ticked, that gives me 1 ring before going to voicemail. Since i am testing this internally, i had to also tick the internal calls, otherwise it was going to ring its 3 times before going to voicemail.
If you are sending this to another extensions voicemail, enter "#[extension]" w/ [extension] being the extension of the voicemail box you want the calls to forward to.
(i don't know exactly how this would work, but you could try a short-code for a voicemail leave and forward unconditionally to the short-code)