About 80,000 C&P Telephone customers in parts of Reston, Herndon and Centreville may be about to miss a chance to be included in the Washington metropolitan calling area, Del. James H. Dillard II (R-Fairfax) has warned.

Under the proposal, phone calls to Maryland would be considered local instead of long-distance. A 50-cent monthly charge would be added to customers' bills in exchange for the service.

Under state law, a majority of the customers in the exchange areas must mail in ballots on the proposal by this week for it to be approved. A majority of the ballots must be cast in favor of the change. The plan has to be approved by federal, Maryland and Virginia officials before it can go into effect.

As of last week, about 27,600, or about 46 percent, of the customers in the exchange area including Herndon and parts of Reston had returned ballots, Dillard said. In Centreville's Engleside exchange, just more than 8,000 customers, or about 47 percent, had responded.

Even though 80 percent of those responses favored the plan, the number of ballots mailed in has fallen short of the required total, he said.

"Most of the people by now have either sent in the ballots or they've lost them," Dillard said. "It would be a shame to lose out in Reston and Herndon by a lousy 3 percent."

The deadline for receiving the ballots probably will be extended from Tuesday to Friday to allow customers to get replacement ballots, Dillard said. Replacements may be obtained by calling telephone service centers at 876-7 followed by the three digits of the exchange.

The proposed change would apply to the Braddock exchange area. Dillard said the number of ballots cast there is sufficient to meet state requirements.