We learn it at mother's knee: Maryland's area code is 301, Northern Virginia's is 703 and the District of Columbia's is 202.

Except that sometimes they aren't.

That's right, phone fans. The home state pride that wells up in your chest every time you leave a phone message in a faraway city, the care you've devoted over the years to making sure your grandmother dials 301 and not 311, it's all for naught.

Here's one of Washington's least-known facts: If you place a long distance call to any phone in Montgomery, Prince George's, Arlington or Fairfax counties or the city of Alexandria, you can precede the basic seven digits with either the appropriate state area code (301 or 703) or with the D.C. code (202). The call will go through either way.

This is the only metropolis in the country whose suburban phones can be called with either of two area codes. But according to Web Chamberlin, a spokesman for C & P Telephone Company, it doesn't cost either the phone company or the customer any more.

"It's basically a matter of customer convenience," Chamberlin said. "The perception of our customers is that when somebody says they live in Washington, they believe that means the whole metropolitan area. We can't see why our customers -- or their business contacts -- need the harassment of having a call turned back because it was preceded by the wrong area code."

The trick to making it all work, of course, is to be sure that no exchange is used more than once anywhere in the Washington area. C & P has awarded "double area code" service only to the closest tier of suburbs because if the company went any further afield, it would run into conflicts.

"Take Glen Burnie, Md., for example," Chamberlin said. "There's a 544 exchange up there. But there's also a 544 exchange on Capitol Hill." Imagine the chaos if you could dial, 202-544, then four other numbers and get either a Glen Burnie housewife or a Capitol Hill restaurant.

One caution: the days of double area code access for the Washington suburbs are not limitless. "This isn't a forever thing," Chamberlin says. "There may come a day when we run out of unduplicated exchanges. And that's the day this ends."

I say that, in the name of honesty, that day can't come too soon.

As it stands, a milk lobbyist can use the phone to convince a dairy farmer in Oshkosh that he's on top of things in Washington, even though the lobbyist's office might be 14 miles away in Rockville and he might not have gone downtown for a week.

There's a word for that sort of thing. It's "deception."

The Postal Service put an end to the same sort of "extended Washington" fiction this summer. It stripped Montgomery and Prince George's customers of the right to use a "Washington, D.C." zip code number.

The other sort of code should be next.