A bill that would have prevented the telephone company from charging for directory assistance calls in Maryland was watered down in the state Senate today by an amendment allowing consumers 12 free information calls monthly before having to pay.

Oddly, the amendment softening the bill was proposed by the sponsor of the stiff measure, Sen. Arthur Dorman (D-Prince George's), after a meeting he had with lobbyists for the Chesapeake and Potomac Telephone Company.

"What we're doing here is being practical," said Dorman, noting that a study conducted last summer by the Maryland Public Service Commission, the state agency that regulates telephone rates, recommended allowing only three free 411 calls a month.

Twelve calls, Dorman said, "is greater than anywhere else in the country."

"This is extremely difficult for me to understand," said Sen. John Corderman (D-Washington) of Dorman's amendment. "This is the first time I've ever seen a bill come on this floor and commit hara-kiri.

"I think the amendment guts the bill," Corderman said. "It's capitulation by the Senate to have this dictated to us by somebody who does not even have a chair in this body.

Sen Donald P. Hutchinson (D-Baltimore County) said C&P representatives asked him and several other senators to sponsor the 12-call amendment before Dorman agreed to it.

"You can't tell me the telephone company did not know it had suffered a significant defeat it has suffered a significant defeat when this bill came on the floor" with the full ban provision included, Hutchinson said.

Most of the members of the Senate, though appeared to agree with Sen. C. Lawrence Wiser (D-Montgomery), who supported the amendment. "You've got the opportunity to get a bill through without the opposition of the telephone company and without the prospect of a governor's veto," he said.

"Recognize a victory when you see one, and take it," he said.

The issue of charges for 411 calls has taken on an added significance here because of the fate of bills on the issue last year. Then, an obsecure Baltimore delegate, Raymond Dypski, outmaneuvered the telephone company's usually impeccable lobbyists, and won approval of a bill imposing a permanent ban on directory assistance charges.

Gov. Marvin Mandel vetoed Dypski's bill, and signed instead one authored by Dorman that imposed a one-year ban on charges and mandated, a study of the desirability of 411 charges. The study concluded that three free calls wes the appropriate solution.

In other action efforts to prevent the state from building a prison that some opponents say will destroy two east Baltimore neighborhoods were thwarted for the second day in a row.

Today's loss of popponents of the proposed prison on the site of an abandoned Continental Can Co. factory would have been a victory had one senator arrived at a committee meeting on time to vote against the prison proposal.

Sen. Edward J. Mason (R-Allegany) was meeting with constituents who has driven several hours from Western Maryland to see him when the Senate budget and taxation committee split 5 to 5 on a vote to take maney for the prison out of Mandel's proposed budget for next year.

The committee meeting was held earlier than originally scheduled because of unrelated timing conflicts. Had he been at the meeting, Mason said, he would have cast a decisive sixth vote against the prison because "that damm things smells to high heaven. It king of reminds me of racetrack deals.I don't know what's going on, but it's something."

The land the prison would be constructed on is owned by Baltimore developer Morton Sarubin, a cousin of Mandel's chief fund raiser, Irvin Kovens. If the proposal is approved by the legislature, Sarubin would make a profit of between $1.1 million and $2.6 million.

The racetrack deals Mason referred to are legislation in 1971 and 1972 affecting Marlboro Race Track, which figured prominently in the federal political corruption indictment of Mandel and five other men, including Kovens.

Moves to defeat the prison have now been defeated in Senate and House of Delegates budge committees. Opponents have vowed to renew their efforts on the floor, and possiblyfilibuster the state budget bill in the process.