So much for the big bang. The region's new area code dialing regime landed yesterday with a sound more like a thud.

The Chesapeake & Potomac Telephone Co. had warned for weeks that as of yesterday, people calling Northern Virginia from a suburban Maryland exchange, or vice versa, would have to add the three-digit area code to the seven-digit local phone number. The same applies for local calls between the District and the two states.

For many people, though, there was no change at all. Although C&P told everyone to begin using the new system yesterday, the utility switched over only 20 percent of the region's phone exchanges. In a little-publicized decision, the utility said it will phase in the new system over the next week to 10 days, to avoid potential phone gridlock. As a result, most callers who failed to use area codes yesterday were able to put their calls through as usual.

"We wanted to focus all of our customers on one date," explained Rich Ellis, a C&P spokesman.

In recent weeks, homeowners were warned to reprogram the automatic numbers on their burglar alarms. Office workers were urged to repunch their speed-dials. Telecommunications centers at the region's major employers braced for trouble. The new system was touted as the biggest change in phone service since direct-dial long distance.

Most of the 1.9 million C&P customers in the Washington area apparently listened to the company's aggressive advertising: C&P reported that 80 percent of yesterday's callers dialed correctly.

"All in all, we're pretty happy with the way things have been going so far," Ellis said.

John Buchko was a little disappointed, though. Looking for some harmless fun, he punched in his home number in Maryland from his office in Washington yesterday sans area code, expecting to get a stern recorded voice telling him he did wrong.

To his disappointment, the call went through.

"We come in all ready to use it, all educated," said Buchko, who works at Washington Hospital Center. "The only problem is everyone's starting to use it, and no one has to."

Buchko wasn't the only one trying out the new system.

The Bethesda-Chevy Chase Rescue Squad, which answers calls from the District and Maryland, reported receiving several phone calls from District residents who were using the new dialing regime to reprogram their speed-dial phones.

The region's volume of phone calls was up slightly from the usual number, Ellis said, although still in the normal range. The phone company was so sure the new system would run smoothly that it did not bring in extra people, and complaints were no higher than usual, according to Ellis. The complaints included a call to Ginger W. Jones, a customer service representative, from a caller who asked for the name of C&P's president. She wanted to vent her anger about having to pay to have her burglar alarm reprogrammed, a service that usually costs $35 to $50.

A check with several area companies, government agencies and hospitals found only scattered problems with the new system, some of which Ellis said could be due to glitches in customers' internal switchboards. Even the Pentagon, which now must be dialed in the 703 area code, said its phone service was peaceful.

"I expected to get calls this morning," said Mario Valentini, the Potomac Electric Power Co.'s telecommunications services manager, "and I didn't."

C&P said the change is needed because the growth in requests for new telephone numbers -- for telecopiers, pagers, car phones and more traditional uses -- is creating a shortage of unassigned numbers.

Under the new system, the phone company can assign the same prefix in all three jurisdictions, although that will not happen "anytime in the near future," Ellis said. For now, people who dial across state lines without an area code will receive a recording telling them how to dial correctly.

"We're going to keep them on until we feel confident that people are using the new dialing pattern," he said. Staff writer Margie Quimpo contributed to this report.