WASHINGTON -- When the Philadelphia suburbs got a new area code four years ago, employees from Donald Culp's security-service company had to visit hundreds of homes to reprogram alarm systems.
Although the phone company indicated the move would create enough new telephone numbers to satisfy demand until 2021, the Philadelphia region will need two more area codes in June. For Culp, that means another round of service calls.
The pattern is repeated across the nation, with the industry running out of phone numbers faster than ever imagined. Growing demand for cellular phones, pagers and second lines for modems and fax machines is often blamed. But more at fault is the way that numbers are assigned, in blocks of 10,000, which leave many numbers unused.
"What's going to happen in five years?" asked Culp, president of Delco Alarm Systems in Aston, Pa.
The rapid depletion of phone numbers is prompting federal officials to find more efficient ways to dole out numbers. The Federal Communications Commission is expected to propose new rules in the coming year.
California went from 13 area codes in early 1997 to the current 23 and is projected to have at least 39 by 2001. The Chicago area went from two codes to five in 1996 and is getting at least one more. Dallas and Houston, which got new codes less than two years ago, are each getting a third code in 1999.
Every change forces business and residential customers to learn new dialing patterns, and many must upgrade equipment and reprint business cards, stationery and advertising.
"Five years ago, maybe, there was a very slow increase in the number of area codes being assigned," said Bruce Armstrong of the Colorado Public Utilities Commission. "Then all of a sudden it started snowballing."
The system, which also covers Canada and the Caribbean, began with 78 area codes for the United States in 1947. Forty-eight codes were gradually added through 1994.
Then it sped up: 14 more codes in 1995, 11 in 1996, 32 in 1997 and 22 in 1998. Industry officials project the need for 30 new codes a year unless changes are made.
Under the block system, created in the days of the telephone monopoly, competing local carriers require a block of numbers for every billing region they wish to serve. An area code may cover dozens or hundreds of such regions.
If a carrier has only 100 customers in a given region, the remaining 9,900 numbers of the block are tied up. The amount of unusable numbers grows if a startup carrier serves several regions or if a region has several such carriers.
"In a monopoly world, where there's essentially one provider, the system worked reasonably well," said Alan Hasselwander, chairman of the North American Numbering Council federal advisory panel.
As few as half the nearly 8 million number combinations for each area code are actually assigned before a new area code is requested.
"We have 1.2 million people in Maine, and they could all have second lines, pagers, cellular phones and who knows what -- and still not have all the numbers used up," said Phil Lindley of the Maine Public Utilities Commission.
But because many of those numbers are tied up -- and thousands more will remain unused as dozens of new carriers await certification -- Maine is considering a second area code.
Federal officials are looking into such changes as assigning numbers in smaller blocks and consolidating billing regions to reduce the need for blocks. Illinois and New York have ongoing tests for small-block assignments, and Colorado recently reduced the number of billing centers by 60 percent.
These efforts can begin "to treat phone numbers as a scarce public resource rather than the private property of the phone companies," said Martin Cohen of the consumer group, Illinois Citizens Utility Board.
But revamping the system could require computer and wiring changes in the phone network and affect 911 emergency systems that use the incoming phone number to pinpoint location.
The industry agrees a change is needed, but various segments -- from longtime carriers to startup carriers -- differ on how to do so.
Details about the phone numbering system:
The North American Numbering Plan governs phone number assignments for the United States, Canada and the Caribbean. The basic phone number has 10 digits, including the area code. Some areas require the dialing of "1" before the 10 digits.
THE AREA CODE: The first three digits. The United States has 205 area codes, plus special codes for toll-free and other calls. Canada has 20 and the Caribbean nations have 21. Before 1995, all area codes had a "0" or a "1" in the middle digit in order to let phone equipment recognize the number as an area code. That restriction was lifted in 1995 because the industry was running out of combinations. The first digit of the area code is never a "0" or a "1."
THE PREFIX, or exchange: The middle three digits. These digits tell computer and switching equipment how to route a call to a specific central office within an area code. The numbers also help with billing, as two billing regions may not have the same prefix. Each area code has 792 prefix combinations. The first digit is never a "0" or a "1," and combinations that end in "11," such as 911, are reserved.
THE LINE NUMBER: The last four digits. There are 10,000 combinations for each prefix, and these numbers route the call to the specific telephone user.