In the Calls app settings, you will find the call forwarding settings which generally control forwarding to voicemail when the phone is busy, unanswered or out-of-service. Of these three conditions, the unanswered condition has an optional delay parameter, which is not implemented in GOS.
People frequently ask "How can I change the number of rings before a call rolls over to voicemail?" You do it by settings the number of seconds delay before triggering the unanswered condition. If you can't do it in the settings menu, you have to do it via GSM "Hash Codes" (which are a PITA).
I've been trying to get the developers to add this optional parameter to the forwarding menu, without much success, so far.