A public IP for every customer would get expensive after a while although it is how the major DSL companies do business. They set up a pppoe (or oA but whatever…) connection and then the modem draws a public IP from a DHCP pool. This is easier for management of users and it makes it easier for you track any problems with customers. I suspect it will make CALEA compliance much easier too.
The majority of people though just NAT their customers and then give them public IP’s on request. We run it this way, and have no issues with any major USA VoIP providers. Any VoIP provider that has issues with NAT is crap in my book anyway, because the VoIP adapter should be able to deal with NAT on its own… Even voip adapters operating through double NAT work fine.