No, it's not! Utilizing your main incoming 800 number and the URL of your home page in your TV commercials is not only shortsighted, it's actively counterproductive. You need to be able to track how many people respond to your TV commercial specifically.
The only way to do this is to set up a separate toll-free number, and a special URL on your law firm's website that relates to the information in your TV ad. Otherwise, your only hope of figuring out whether your TV campaign was successful would be to have a staff member actively debrief prospective clients during incoming calls (“How exactly did you find out about our firm?”), which can be an unpleasant experience for the person on the other end of the line.
It's true that extra expenses are involved in setting up a brand-new toll-free number, and designing a separate page of your site devoted to TV viewers—but then again, these costs are an order of magnitude less than what you've already spent on your law firm TV campaign. All of that money you invested in your commercial, and in buying time from your local basic cable ad rep, will be wasted if you can't track exactly how people respond to your TV ad, what time of day they call you, what they ask you about, etc.
Not that you're interested in hearing about even more expenditures right now, but you also have to invest in good client-management software; if a person is interested enough to call your law firm in the first place, you want to be able to follow up with him on a regular basis!