SOME PEOPLE may not like it, but the local telephone companies in New York City and Los Angeles this fall are allowing their customers to engage in "permissive dialing." No, it's not what you might think; permissive dialing means that until Jan. 1, 1985, you don't have to use the new area codes for the portions of those metropolitan areas that have been assigned new prefixes. But the phone company would like you to try, and after the New Year, if you want to reach a friend in the San Fernando Valley or Flatbush, you're going to have to dial 818 or 718, not 213 or 212.

The reason is simple. Area codes 212 and 213, covering until now all of New York City and Los Angeles County, ran out of numbers. And they're not the only ones that have. The hinterland of southeast Texas has a new code (409) because Houston's (713) was runnng out of numbers. Orange County remains 714, but San Diego and Palm Springs a couple of years back became 619. Within memory of many in these parts, southside and Tidewater Virginia were cut loose from 703 and given 804.

There are almost 10 million possible phone numbers in each area code. But as businesses expand, as Centrex arrangements proliferate, as the young and the elderly establish their own households- cum-telephone, local phone companies are running out of digits. The Reagan administration may cite this as an example of the surging economic recovery. Its critics may decry the break from tradition when 714 becomes 619 and ponder the status anxiety of Brookl-known 212 and relegated to an unknown 718. AT&T will undoubtedly deny that divestiture has anything to do with it. But what are the phone companies going to do in what suddenly seems like that not-so-distant day when North America (Canada, Mexico and the Caribbean have three-digit area codes too) runs out of numbers?