I know it's a lot of work, but Mr. Warren's advice is excellent.
Another poster wrote:
Many years ago this happened to us, except it was our calls being billed to someone else. (We reported it to cover our butts). A few months later they discovered the error. The billed party, a business customer, had been screaming about the calls, but the phone co at first refused to do anything since they claimed it could've been any employee. Finally they did a physical check in the CO and found the wiring error.
In thinking about it, I wonder if the business customer got billed for our message units as well. We had message rate service though we were pretty careful to stay within the monthly allotment. (Flash forward to today -- we went to flat rate service, then Metropolitan Area flat rate, and now have national unlimited.)