Workers building a Virginia Electric and Power Co. power plant in the western part of the state make up to $20,000 a month in unmonitored personal phone calls on company phones, an angry witness charged here yesterday.

Ann Marks told the three-member State Corporation Commission in public hearings on Vepco's $246 million rate increase request that the phone calls were only one example of waste and inefficiency at the plant site that are passed on to customers.

Vepco's new rates, if approved by the commission, will raise electric bills by more than 20 percent for 1.2 million customers.

Marks also said that a subcontractor burned or buried the equivalent of 324,000 board feet of valuable timber and 57,700 cubic feet of pulpwood cleared from the Bath County plant site that could have been sold.

"I tried to purchase walnut trees in the lower valley," she said. "I was informed . . . that the trees were not for sale."

Marks, a Highland County resident, cited other instances of alleged waste by employes of Daniel Contruction Co., Vepco's general contractor on the job, and by subcontractors. Representatives of Daniel could not be reached last night for comment.

Vepco executive vice president William W. Berry said later he does not think there is "any real substance" to Marks' allegations, but he said that the FBI is now "checking" another allegation raised by Marks: that a worker on the site ordered $30,000 in unneeded bolts from a friend and was fired for doing so.

The $751 million Bath County plant, scheduled for completion by 1983, will consist of two lakes. Electricity will be used to pump water from the lower lake to the higher one at night when the company has spare generating capacity. Then, during the day when there is a high demand for electricity, the water will run back downhill, activating generators.

In a related development, Sen. Gary Hard (D-Colo.) said "that it will be difficult, if not impossible" to determine finally the date that the Nuclear Regulatory Commission learned that a geological fault existed under the site of Vepco's North Anna nuclear generating plant.

Releasing a study he requested last fall from the commission, "the report indicates that the commission was aware of a serious geological anomaly . . . and failed to act expeditiously."