I Know It

R.H. Donnelley: An Attractive Bet for High-Risk Investors- Barron's

Most YP companies can set up call tracking for advertisers. This enables the advertiser to know the calls were generated by the YP ad - even down to which book consumers are using.
Or - the doc may do it the old fashion way. He may ask his Admin Asst. to simply ask the caller, "How did you find us?" Then they simply keep a tally of which type of advertising generates the most ads. It is not that difficult to do and most smart business owners will do this.