So you were wondering about a price for it as an already up-and-running turnkey site that someone can take over as-is, but they'll need to get the number on their own if they want to turn it into a business.
I see value in it. Your site is nice and clear, and provides a service that's popular and valuable these days.
I'm guessing, after only spending moments on your site; but it's not a business, right? Just a portal with some well-monetized ppc links? Big difference of course, since buying the site as a portal vs. buying it as a business makes a huge difference.
If you just want to sell kit & kaboodle portal, and someone has no intention of getting the 1-800 number, it's a simple deal; if you provide stats for your monthly ppc income, either here or in pm to anyone who requests, it's standard to sell your dn at the average monthly income it brings you x (times) a number of months; that number depends on so many factors that it's up to you and the buyer to negotiate. Anywhere from a few months' revenue to a couple years. Depends on how well the site is doing, or how well the buyer thinks he can increase it.
All this was assuming, of course, what I think you wanted: to sell an already-running portal, as-is.
Still no value if selling as just a dn with no development.