Google Voice actually goes to voicemail based on time, not rings.
From Google themselves:
If you don't respond to a call within 25 seconds (by accepting, sending to voicemail, listening in on voicemail, or accepting and recording the call), Google Voice will send the caller to Google voicemail.
To prevent your mobile phone, home, or work voicemail systems from recording dead air or the Google Voice prompts, simply make sure that ringing your phones and connecting to your outgoing message on those systems takes longer than 25 seconds. Most providers add at least five to 10 seconds of instructions at the end of your personal greeting. Keep that in mind when timing your mobile phone voicemail greeting.
If you've turned off Call Screening, we'll send the call to the numbers you're forwarding to, and your caller will be connected to your home, work, or mobile phone voicemail systems or answering machines if these phones' voicemail systems or answering machines pick up before 25 seconds. If these voicemail systems or answering machines don't pick up before 25 seconds, the caller will be connected to Google voicemail.
