ip accounting could be used to track the traffic via ip....this is available on the router already but perhaps not very 'clean' to get a report out of.
for billing, i use InforTel from ISI:
http://www.isi-info.com/solutions/solutions_select.htm
there are many packages out there, perhaps some freeWare. type in "call accounting" in your search engine without the quotes to see many, many options. (you can also use "freeware call accounting" to get a list of any freebie packages)
