The first toll-free phone lines, called WATS lines (for Wide Area Telephone Service), were introduced in the United States nearly 50 years ago. For years thereafter, all toll-free numbers used the prefix “800,” so that many consumers came to refer to toll-free lines as “800 numbers.” And they were very popular with consumers because of the then-relatively high cost of long-distance calling.

But just as the rise of cell phone popularity caused a proliferation of new area codes, the growing popularity of toll-free phone numbers meant a dwindling supply of lines within the “800” prefix. Hence, the introduction of “888,” “877” and “866” toll-free prefixes have been made over the past 13 years to expand the supply of available lines.

But old habits die hard. Even today, many consumers reflexively refer to all toll-free lines as “800 numbers.” And indeed, a study conducted earlier this year by Engine Ready, a California-based search marketing software and service firm, finds that “800” lines actually outperform the other prefixes when it comes to phone conversions.

For the study, Engine Ready sampled ~18,000 visits to a single lead-generation web site. The visits were driven by a Google AdWords search engine marketing campaign, producing ~2,600 call-in and online conversions. Visits were split evenly among four web landing pages that were identical save for the call-in response action that contained distinct phone numbers featuring the four different toll-free prefixes.

While little difference was observed between the four prefixes in online conversion behavior (form fills), the “800” prefix clearly performed best of the four toll-free lines for call-in responses. Its conversion performance ranged from 20% to 60% better than the three other phone lines — that despite the fact that there was no practical difference at all between the phone numbers except for the different prefixes.

Moral of the story: Even in today’s “the only thing that’s constant is change” environment, sticking with the “tried-and-true” when it’s possible to do so can be a pretty smart move. And if it’s inbound telephone sales you’re doing, make sure you insist on getting one of those old-fashioned “800 numbers.”