The paper said that the investigators were trying to find out whether MCI had disguised long distance calls as local ones, so as to avoid paying fees to other phone firms.
The BBC article lists no amounts, but my local paper mentioned a figure with 8 zeroes. (Just found another link at [link|http://www.msnbc.com/news/944683.asp?cp1=1|MSNBC]. This one figures there may be another $1B skeleton falling out of the closet)