Maryland's public service commission yesterday approved a $19.1 million annual increase in Chesapeake & Potomac Telephone Co. rates including a boost from 10 cents to 15 cents for pay telephone calls.

The rate hike approved for C&P the second in Maryland in the last 14 months, was less than one-quarter of the total amount sought by the telephone company, a subsidiary of American Telephone & Telegraph Co.

C&P had proposed an $82.3 million increase last Nov. 5, citing soaring costs for needed modernization and expansion of facilities. The telephone firm also sought a 20-cent pay call rate based on AT&T data showing that the average pay telephone call costs the firm 16 cents.

A public service commission announcement said the higher rates could take effect immediately, but a C&P spokesman said last night it will take 10 days or more to file new tariffs and covert pay telephones. For typical residential customers, yesterday's decision will mean an increase of 25 cents a month in their bills.

The top officer of Maryland C&P, vice president Thomas M. Gibbons, said in Baltimore that his firm is "sorely disappointed by the decision (that) simply ignores the economic facts of life of our industry . . . the increase . . . is helpful but still unrealistic under today's economic conditions."

Another C&P spokesman said last night the commission action "will be evaluated carefully . . . with this kind increases in basic monthly business of decision it will keep us in the business of (seeking) rate increases," a hint that Maryland C&P soon will propose an additional rate boost.

In February 1976, the Maryland commission approved a $20.6 million rate hike for C&P, the first increase in basic telephone rates in four years.

Among provisions in yesterday's commission order:

C&P's rate of return - the amount of profit permitted on its total plant and equipment investment in the state - was increased from 8.8 to 9.25 per cent.

A proposal to increase service connection charges was denied.

C&P was allowed to begin installing equipment for timing business calls to make it possible to charge customers by message until in the future.

After the facilities are in place, C&P must proposed new business rates; no charges were sought by C&P in the current case.

Long-distance rates within Maryland may be increased by no more than 5 per cent.

The pay phone increase is the first in Maryland in 25 years. The District rate also is 15 cents and the pay phone rate in Virginia is 20 cents.