That's not possible. And how long your phone rings has nothing to do with having voicemail or not having voicemail. Your carrier controls how long your phone rings. Most carriers can extend the ring to 30 seconds. Whether your carrier can extend it beyond 30 seconds is something you'd need to discuss with them. Finally, what good would it be for your phone to ring forever? I can tell you that if after 30 seconds, the person I'm calling doesn't answer and the phone doesn't go to voicemail, I'll just hang up. Without voicemail, you'll need to check your Recents list in the Phone App to see who may have called you.