I do track every job that comes in.
Here's some of my observations:

The one thing I have found the last few years is the increase in "shoppers" from the yellow pages. They seem to call and just go down the list until they either get the lowest price, or they hear what they want and book the inspection. When I bought my friends HI company, we transferred his phone number to ring into mine. I would get a call, give them the price, and a minute later they would call the other number.

Very few people will leave messages. While we do have a live person answering the phone, there is the times where it does go to the message.

Most of the internet "shoppers" I get are just that. Cyber shoppers that refuse to pick up the phone to talk to a live person. I have a very low close rate with internet shoppers.

The yellow pages do generate lots of calls.
The calls do generate booked jobs.
They do pay for themselves, though they are not huge revenue producers.

While most of my business comes from referrals from past clients, attys, mortgage companies, etc, enough comes from the yellow pages to not drop them altogether.

I have been reducing the size of the ads, but then went into a new book to expand service area (closer to where my guy lives).

In Knoxville, we have two major phone book companies and one wannabee. We have Bellsouth and Yellow Book with Verizon trying to break into the market. I have to be in both books and my service area covers 3 major books and one minor one. I'm in a total of 8 books.

I will continue to be in both books since I see a mixture in the houses I'm inspecting, but will scale down the ads each year. I have only a BOLD listing in some books.

I guess you have to try to determine if your market is yellow page driven or not and act accordingly.
JF