Reverse lookup software depends on information available in databases and data files. The usual way of looking up a phone number is to search for the name of the person or business. The phone number is simply part of that record, either in the text file or in the database. It’s like looking it up in the yellow pages. It simply is there. But such records are ordered alphabetically or even categorically. They aren’t ordered by telephone number. Consequently you can’t efficiently look through the phone book trying to find a random number. It just takes too long in the current order. Humans simply could not do the work in any reasonable time, nor could they be expected to always determine the right information as humans can make unintentional mistakes.

For a computer, however, reordering and searching for a particular phone record is not very taxing. The computer is capable of quickly reviewing hundreds of records and reordering them based on any factor at all. Phone numbers are easy. By quickly comparing the phone numbers they can be reordered numerically to line up with area code, local exchange and finally your four digit extension. The result is something that computer programmers can efficiently program as a lookup table. Even if your database is full of all the ten digit numbers possible in the USA the file would still be completely searched in only 33 steps. Actually, if it were full the file could found in only 1 step. This makes it a great advantage for the computer to spend the time on reordering and sorting phone records.

So the computer based search is only limited by the databases it has accessed. Typically, cell phone numbers are only recorded as cell phone numbers, with only occasional records to indicate who might be on the other line. There are also non-directory numbers which may not be available on any database open for purchase. Most home and business numbers, however, are instantly available with details about the phone’s owner. All of this costs money so the reverse directory company may choose to charge for certain databases. Depending on the amount they were charged for the database access and reverse mobile phone directory may pass on a moderate fee to the consumer to compensate for the expense. In many cases a simple membership with a site will cover any such fees and is an excellent choice for those who regularly engage in this sort of work.

In conclusion, looking up the phone number of an unknown party is specialized work requiring the use of a computer indexing program, database access and an efficient search algorithm. The companies who provide this service can give the average consumer access to this information in order to verify the identity of a caller and for other business specific purposes. For these services they only charge a nominal fee or offer memberships to avoid future fees. In this way the service is maintained and available for the future as often as it may be needed.