Try to answer your phone as quickly as possible for most people if a phone call is going to their cell phone, it's right there it is pretty quick. Some people still like to let the phone ring a whole bunch to somehow, like they're not too eager. I don't think you can really answer a phone too quickly. I don't think there's any negative vibes anyone's going to pick up on that. I would suggest to you many cell phone plans have an option of how many rings it does before it goes to voicemail. You don't want to make someone have to call and go through 1012 rings before it hits the voicemail.
I would say after four rings, if you can't pick it up, let it go to voicemail that's less time that you're taking of the person trying to call you just makes it a lot easier. So I would go ahead and put your setting on that. That's your homework assignment right now call your own phone. See how many rings it takes before your voicemail sets.