One thing the Internet does well is provide access to databases.
For the last 10 years or so, it was possible to get white pages listings from incumbent local telephone companies on line. anywho.com, for instance, started out as a database for Ameritech states. GTE had one, as did each of the baby bells. With consolidation, these databases got bigger.
In the ILEC's service territory, the listings came from their own databases. Now, they might have been the listings as they existed at the time the latest telephone directory closed, but this was decent information, less likely to be out of date. Typically, the database would not have disconnected numbers, even if new movers were missing.
One by one, these databases vanished from the Internet. Generally, Intelius either purchased the domain names but left the front end in place or the domain is still owned by the phone company but supplying Intelius data.
Intelius is a data consolidator, purchasing any database it finds. The information is consolidated with fuzzy matches, so it's hardly unusual that information from multiple people is linked with something in common in the matching criteria. Generally, the results are somewhat useless. Recently I found a phone number briefly assigned to a relative at a location she never ended up moving in to. The phone line wasn't supposed to have been activated (it never terminated at the location) but it was for a short time. At Intelius, it's listed in her name at another address she has no association with. I'm sure the phone number had been assigned to someone at that address either before or after she had it.
Intelius makes its money from paid information searches, so the fuzziest possible matching is their business model. I can't imagine that their paid searches provide better accuracy. In fact I'd imagine it's probably much worse, with more hits on additional databases. I've never used it.
lycos's whowhere.com is a front end to ussearch.com, an Intelius competitor.
The only remaining non-Intelius database is whitepages.com. Most of their information is from databases prepared from printed telephone directories input by third parties, so it's out of date at about the same rate as a telephone directory not published by the local telephone company. Some white pages searches provided by the big phone companies, like Verizon, are actually front ends for a search through their database.
Might as well resume using printed telephone directories. The number of published listings is way down, as the ILEC doesn't carry all telephone numbers from CLEC's, but at least what you find stands a better chance of being accurate.