To amend knarph's writeup, 311 has served many diverse purposes for local telecommunications carriers over the years.

In the 1980s and possibly earlier, dialing this number in a Southern Bell or Southcentral Bell service region (later joined together as BellSouth) of the United States would give you a special computerized function of the local NOC that verbally identified the seven digit telephone number of the line you were dialing from. This worked mainly for POTS circuits, and was sadly phased out in most areas following the federal telecom deregulation legislation of the 1990s.

Knowledge of this feature was something of a closely-held secret among phone company employees, and was mainly used by outside line workers and contractors. The need for this feature has not changed, but the number that now must be dialed to access it has. In most cases, it is now a regular seven digit number, which is changed periodically by some local telcos in an ongoing effort to keep it secret.

In Tallahassee Florida where I live, which is served by Sprint as the local telco provider, the number to dial for circuit ID is 520-3111. It is interesting to me that they still keep the 311 component of the phone number, but other telcos don't always do this (given that I know several for different cities in the southeastern US).

It should be noted that dialing this number long distance will not work - it must be dialed from a local telephone. If you call your operator (dial "0") and attempt to represent yourself as a phone company employee or some other telecom worker, there is a 95% chance that they will not give you the number for your area that performs the circuit ID function. However, in today's world of digital cell phones with Caller ID, this function can often be achieved with the local telco's silent acquiesence. Muahaha!