First, it is interesting that we are discussing phone book advertising. For most shops, this form of advertising is not effective, since the increase in online searches overshadows any other form of traditional advertising.
As with any form of advertising, the best way to track the effectiveness is to ask all first time customers how they found you. But the key is ask a follow up question; "In addition to the radio ad you heard, has anyone else referred you to us?" Many times the customer will say, "Yes, my neighbor Tom Smith recommended you, and I also pass by your shop every day."
Tracking phone calls is good, but unless you turn the call into an appointment, you will not know the true picture.
Lastly, you must track these first time customers and get information. And ask a lot of probing questions. Hopefully, your management software system has a field you can enter the information. If it does, you can generate a report for all first time customers. If not, create your own system.
This type of customer interview also helps to establish a relationship.
I hope this helps.