The nation is rapidly running out of telephone numbers.
As facsimile machines, cellular phones, pocket pagers, computer modems and business telephones proliferate, the supply of unassigned numbers is being exhausted in the area codes serving the most populous areas, such as 212 for Manhattan and the Bronx, 213 for Los Angeles and 301 for Maryland.
When that happened in the past, an area code was split into two, based on geography. In fact, starting this fall, Maryland will phase in a second area code -- 410.
But now only three unallocated area codes remain: 210, 810 and 910, and they may not be enough to satisfy the booming demand.
Yet they must last until July 1995, when a new numbering plan is scheduled to take effect, telephone industry executives say. The expansion cannot take place until then because all the nation's telephone switches must be reprogrammed -- a complex and time-consuming task.
"You have four full years to go," said Frank J. Saletel, director of major-market access planning for New York Telephone Co. "There could be some areas of the country that may have to resort to rationing their delivery of numbers."
The change in the numbering system planned by Bellcore, the telephone industry research organization responsible for allocating phone numbers in North America, would permit the creation of area codes that do not have a "0" or a "1" as the middle digit.
Computerized telephone switching equipment currently looks at the second digit of an area code to determine whether a long-distance call is being made.
Some telephone industry executives say the needed changes may be costly and frustrating and would require many callers to dial 11 digits, rather than seven, even when calling a neighbor in the same area code. They also say the change would be difficult for the telephone companies and would create serious problems for businesses.
Further confusion would be created because of differences in how toll calls are dialed within area codes.
Bellcore cannot enforce standards but can only recommend changes to individual state public utility commissions.
So far, the Bellcore plan has been installed in 23 area codes, and about half of those require callers to dial "1" plus their own area code before the number, while the others just have to dial the number.
A number of alternatives are being proposed.
Some industry people are backing a proposal that would increase the local dialing sequence to eight digits from the current seven, and several smaller telephone companies have proposed a four-digit area code.
No serious consideration is being given to the idea of combining lightly used area codes to provide more numbers to congested areas.
"Like lambs to the slaughter, American telephone users are being led into a course that will have far worse implications than what most of us see at the moment," said David C. Henny, president of the Whidbey Telephone Co. in Langley, Wash.
In a technical industry journal, Mr. Henny recently proposed a system of four-digit area codes, which he says would be simpler than the Bellcore plan and would provide a 10-fold increase in area codes.
He says his plan would require fewer changes in telephone switching systems and would result in numbers that would be easier for people to remember.
But Bellcore officials argue that their plan is the most efficient and inexpensive and that their system would last well into the next century.
They also say the alternatives probably would cause more confusion or would be more expensive because every telephone customer would have a new number.
Such a switch would force businesses across the country to change stationery and business cards, among other adjustments.
Bellcore's area code plan would increase the potential number of phone numbers to approximately 6 billion. The plan would add 640 area codes to the current supply of 152.
These area codes allow a potential of 900 million numbers, with about 250 million numbers in use now. Within each area code, there are 792 available prefixes, consisting of 10,000 numbers.
Area codes and prefixes that begin with 0 or 1 are avoided, and some numbers such as "800" and "900" are reserved for special uses.
The plan would also create overlapping area codes. Rather than carve Manhattan into two geographic areas with different codes, in January New York Telephone will add a second area code, 917, to overlap Manhattan's existing 212 dialing area.
The 917 code initially will be used for assigning new numbers for cellular phones, pagers and New York Telephone's internal operations, but officials say they may eventually distribute numbers in the new area code to computer users and regular telephone customers.
This model, in which one geographic region will have a pool of two or more area codes, will ultimately require callers to dial 11 digits for both local and long-distance calls. For example, a Manhattan resident with a 212 area code might have a next-door neighbor in the 917 area.
The Bellcore officials prefer a standard, nationwide 11-digit dialing sequence to avoid different dialing procedures in different parts of the country.
"People are going to get used to 10-digit numbers," said Ron Conners, the Bellcore district administrator in charge of the North American numbering plan. "Psychological studies have shown people don't care what they have to dial, as long as it's consistent."
Bellcore executives say that they do not expect opposition anything like the anti-digit dialing movement that greeted the transition from traditionally named exchanges such as PLaza and PEnnsylvania in the 1950s in many urban areas.
"I think we can pretty much say that the United States has the best numbering plan in the world," Mr. Conners said.