Calls to the voice mail system are taken from your monthly minute allowance. With Visual Voice Mail, of course, that's a non-issue.
I rarely hear anyone complaining about running out of minutes any more. Many people have completely replaced their home phones with cell phones (I'm one) and have plans with more minutes than we'll ever use, rollover minutes, etc.
2 iPhones on our plan with AT&T. 1400 minutes a month, shared, 10 numbers we can call for free. Free nights and weekends. 300MB data on my wife's phone, 2GB a month on mine, 1000 text messages a month on hers, 200 a month on mine... costs us about $150 a month and we never go over. We could probably drop it down, but there are months when we come close, so it's not worth the bother.