From Ancestry.com Wiki
Telephone directories are the descendants of city directories, with the criterion for inclusion simply being subscription to the phone service. Telephone directories are useful as locators in place and time—but primarily for twentieth-century research. However, they won’t include such useful information as occupation or the names of spouses and children—unless the subscriber has requested and paid for this information to be included. Moreover, there are always unlisted numbers and persons who do not wish their addresses or given names to be printed even if their phone numbers are. The usefulness of telephone directories in genealogy is, therefore, somewhat limited.
Nevertheless, a book “with no plot, but a cast of thousands” can’t be all bad. When used in conjunction with other sources, telephone directories can help to locate living distant relatives or modern successors to nineteenth-century churches and funeral homes.
In the early 1990s I was contacted by a man from Australia who was seeking his father—an American sailor who had been stationed there in the closing days of World War II. The client had only his father’s name and the name of the ship on which his father had served. Through naval muster rolls available at the National Archives, it was determined that his father had enlisted in a certain Midwestern city. City directories for that city in 1945 and 1946 showed the client’s father living with his parents and a brother who was also in the navy, but their names disappeared from the city directories in the 1950s without a clue as to where they had gone. The Social Security Death Index revealed the death of the brother on the West Coast in the early 1970s, and his death certificate indicated that the client’s father was then living in New England—but no one of that name listed with a New England address in PhoneDisk USA, a nationwide telephone database on CD was the right man.
While the surname was not as common as Smith, for example, and the given names were relatively uncommon, dozens of entries for the names of the client’s father and brother were found in PhoneDisk USA. Nevertheless, letters and self-addressed, stamped envelopes were sent to each of the addresses. Some were returned as undeliverable—the individual had moved or died—and many were returned by the addressees, indicating no knowledge of the persons named in the letter, which had been carefully worded to avoid a negative response if the client’s father did not wish to be found.
At last, a response was received from the client’s father. He was among those listed in PhoneDisk USA and was living in a Southern state. Without this current nationwide telephone directory, locating the client’s father would have been like looking for a needle in a haystack.
PhoneDisk USA and similar CD-ROM telephone directories are becoming obsolete because of the wide availability of online telephone databases. Not all of these databases provide e-mail addresses, and in cases where contacting a long-lost relative may be a sensitive matter (especially in cases of adoption), the old-fashioned letter with a S.A.S.E. may be more advisable.