Chesapeake & Potomac Telephone Co. asked yesterday for steep increases in telephone rates for Virginia residents in an effort to boost its revenues by $133.5 million, or 15 percent, a year.

If the Virginia State Corporation Commission approves the request, most Northern Virginia residents would see their monthly phone bills increase by at least 21 percent.

The $11.79 monthly fee residential customers now pay for unlimited calls in the D.C. metropolitan area would increase to $14.06 under C&P's proposal.

C&P's monthly leasing charge for its equipment also would increase -- by 61 cents for each push-button phone and by 45 cents for each rotary-dial phone a residential customer has.

Installation costs also would rise -- by 28 percent or more in some cases -- under a new pricing formula C&P wants to impose.

Maintenance visits to repair equipment not furnished by C&P would increase by 46 percent for residential customers and by 87 percent for business customers.

The requested increase comes less than a year after the Virginia SCC granted C&P permission to impose a 10 percent surcharge on Virginia phone bills.

Just last month, C&P asked the Maryland Public Service Commission for a 27 percent increase in rates, even though the company had received permission in March to boost its rates by more than 13 percent. In April, C&P won permission to raise its rates for almost all District residents by 19.6 percent.

For Virginia residents, C&P is asking for:

* A 19 percent increase in the unlimited monthly service that is used by 80 percent of Northern Virginians, from $11.79 a month to $14.06.

* A 19 percent increase in the message-unit service, which permits customers to make 50 calls a month in the metropolitan area, from $8.25 a month to $9.84. For each call above the 50 limit, customers would be charged a higher message-unit cost, from 10 cents a call to 11.9 cents.

* A $2.99 increase for each business phone, from $15.66 to $18.55 a month, accounting for a 19 percent increase.

A new 30-cent service charge for operator verification of an apparently busy line, plus an additional charge of 50 cents for interrupting a call. No charge would be imposed if the line was found to be out of order.

* A $3 service charge for person-to-person long-distance calls; a $1.55 service charge for operated-assisted, station-to-station long-distance calls other than on credit cards; and a 60-cent service charge for station-to-station credit-card calls.

* A new formula for installation charges that will boost the fee a customer now pays for installing a single phone in a new house from about $51.38 to $65.80. Installation fees for business customers will increase from $82.39 a line to about $97.65 a month.

* The maintenance-visit charge would increased from $30 to $43.80 for any trouble that was caused by a non-C&P telephone.