The original goal of this interstate call pricing scheme was to help encourage universal POTS service through America without creating something like the Universal Service Fund, primarily leaving it to "free" enterprise to provide universal coverage, mainly through small, rural telco cooperatives in areas Ma Bell wouldn't service.
Eventually the USF was created to help the least connected areas that didn't have tons of urbanites calling to fund their local telco cooperative. Meanwhile, these interstate calling rates were layered with intrastate calling rates as the baby bells prepared for local loop unbundling, expecting competing telephone companies to mainly target consumers.
Instead, what happened is CLECs mainly focused on business, creating those classic 1000 free hour discs from AOL, NetZero and the like. This pissed the Regional Bell Companies (which were the fragments of Bell Systems) off quite a bit, and they've been pushing forward with bill and keep as much as they can ever since.
Bill and Keep is essentially we bill eachother, and its hopefully close enough to a wash that no checks need to be cut. Doesn't work out that way though when dealing with even the most benign of businesses as your primary customer, plumbers, pharmacies, medical practices and the like mainly take inbound calls.