For almost one year now, Claudia Clemmons, a computer software technician at Chesapeake & Potomac Telephone Cos., has been putting in at least eight hours of overtime a week in preparation for Monday, when people in the Washington area will have to start dialing area codes when they make local calls to another jurisdiction.

"The money was really nice for a while," said Clemmons.

But then after several months it became "Oh God, I wish this were over so I'd have time for myself."

Clemmons's job is huge indeed. She and other technicians are typing new instructions into the huge computers that control C&P's area phone system to reteach it how to route the nearly 8 million calls that area residents make each day.

It used to be that human operators connected calls by plugging in wires on a switchboard. Today, computers do the equivalent, listening to the electronic tones that phones produce when a number is dialed and, after consulting instructions like those Clemmons is putting in, open and close switches to send calls to their proper destinations.

Until Monday, the network still will know where to send a local call based on the three-digit prefix of the number dialed. But starting Monday, it may not know what to do with the call unless the caller first dials the area code when calling from Virginia to Maryland, or D.C. to Virginia, for instance.

Calls within an area code -- from Rockville to Bethesda, for example -- still will require only seven digits. Local rates will continue to apply to all local calls, even when area codes are used.

C&P is launching a massive advertising campaign to get customers accustomed to the idea of using the area code. One of the new computer instructions that Clemmons is typing will give callers a recorded message reminding them to use the area code if they forget to include it.

Irene Green, an engineer who supervises the 20 to 30 people involved in the changeover, said the job required her to put in 10 more hours of work each week.

"You came home late last night," became a common refrain of her 6-year-old son. Even at home, she said, she sometimes cannot pay attention to her children because she does most of her overtime work from home, calling different C&P switching centers to check for problems. "Sometimes it's inconvenient but it's got to be done," she said.

Preparation for the 10-digit dialing began two years ago. For the first six months of the project, officials spread across 82 departments of C&P sat in numerous meetings to plan the changes that would need to be made. With each department contributing at various times about four to five people, the number of C&P personnel who worked on the changes totaled 300 to 400.

"Hundreds of people were involved, although no specific employees were pooled for the 10-digit dialing. Everyone did the work within their schedules," said Michel Daley, C&P spokesman. He declined to say how much was spent on overtime pay and on the advertising campaign.

C&P says the change is necessary because the fast-growing Washington area soon will run out of numbers. By requiring area codes, C&P will be able to assign three different customers -- one in D.C., one in Maryland, one in Virginia -- the same seven-digit number.

Stephen R. Fleming of Northern Telecom Inc., a major telephone-equipment manufacturer, used a file cabinet analogy to explain what C&P is doing. File folders with different labels represent phone numbers. At present, the file folders are all in one drawer of the cabinet, whether they are for D.C., Maryland or Virginia.

However, as the files multiply in number, three folders with the same label can crop up. To differentiate them, each has to be put into a separate drawer. If a person has the drawer number and the file label, an individual file can be found.

In the same way, phone numbers in the area have multiplied. Just as drawer numbers are needed if a person is to find a certain file, area codes are required if a call is to find its way to the correct telephone.

The first phase of the 10-digit dialing plan was the implementation of so-called "permissive" dialing, meaning that phone lines in the area were taught to accept both 10-digit and seven-digit calls for local calls across state lines, so as to give people some practice in the new system.

That change was made last January in Northern Virginia and by April was in effect in the District and Maryland. By Monday, however, each switching center will be electronically directed to accept only 10-digit calls for calls that cross area codes.

C&P service representatives, who take care of phone requests, transfers and other queries, have not received many calls about 10-digit dialing. Nor do most of them expect an increase in the volume of callers on Monday.

Company officials also said they probably won't need any additional personnel on Monday since most of the work will be done by the recorded messages.

Gary C. Clark, manager of C&P's Residence Service Center No. 1, said the new system hasn't generated many calls from customers. "No customer has called to ask specifically about the change. We don't expect a barrage of calls," he said.