The District Public Service Commission yesterday approved an overall rate decrease for the city's residential and business telephone customers that could cut average monthly telephone bills by 67 cents.

The commission approved a $10.9 million rate increase for Chesapeake & Potomac Telephone Co., about $15 million less than the city's phone company sought when it initially asked for the rate hike in 1979. The requested rate increase would have brought C&P additional net revenues of $18 million.

Although the commission is not expected to issue a formal decision on the matter until today, the action was approved during a brief meeting yesterday.

The reduction is business and consumer rates means that the bulk of the increase will be carried by institutions that use sophisticated equipment, such as switchboards.

In addition, a spokesman for the company said the increase also would come in higher rates for installation, connection and other charges which are separated from monthly phone costs.The commission approved an increase in the company's rate of return (the level of profits related to total investment in providing local service) from 9.45 percent to 10.66 present.

The spokesman said the company would await receipt of the decision before deciding whether to accept it or appeal it back to the commission. No effective date for the decision will be known until the final order is issued.

Brian Lederer, the District people's counsel, said the decision appears to indicate that the commission is willing to "hold the line" on consumer rate increases at a time when telephone regulation is loosening across the country.

Lederer, whose office fought the rate increase, said he did not believe the company "should have gotten any rate increase.