#1 Enterprise level billing systems for internet providers cost a lot of money. The more regulations/rules/requirements the system has, the (likely) more expensive it will need to be. In my past experience we "rolled our own", but, this isn't feasible for most in the arena for many reasons and in hindsight it was a bad idea for liability reasons.
Why would a small business need enterprise level software?
There are a few providers that handle this for reasonable fees. We used Platapus for the small ISP I worked for.