In an effort to cut Uncle Sam's multimillion-dollar telephone bill, federal agencies began electronically blocking 145,000 of the 230,000 phones in government offices here this week to make it impossible for workers to dial time, weather or other prerecorded messages such as Dial-a-Joke.

Each call costs the agencies between 6.9 cents and $1, and cutting back on them could save about $300,000 a year, a government spokesman said.

Workers in most federal offices in New York and Chicago have been unable to make such calls since last year, when such a system was instituted there. The two cities were picked because of the high concentration of federal employes and their relatively sophisticated phone systems, federal officials said.

The General Services Administration plans to extend the telephone curb to another 26,000 federal phones here later this year. The U.S. government was billed $34 million for calls made within agencies in this area last year; calls from agencies to the outside cost another $6 million, including about $250,000 for weather and time checks and $40,000 for calls to recorded messages, a GSA spokesman said yesterday. The government is charged 6.9 cents in the District, 10 cents in Virginia and 11 cents in Maryland for every outside call made by an agency, said Don Hardesty, regional director for GSA's Office of Information Resources Management.

Eighty-six percent of all federal telephones here are on an electronic system that can block types of calls, Hardesty said. Major departments such as Treasury, Justice, Commerce and Energy are on the system.

GSA plans to extend the blockage to the National Aeronautics and Space Administration in Greenbelt and the National Institutes of Health and Nuclear Regulatory Commission in Bethesda, the spokesman said.

Agencies outside of the GSA telephone network are expected to follow suit if they have the capability to block calls to certain numbers, he added.

The weather and time numbers are called by thousands daily, the Chesapeake and Potomac Telephone Co. said. When snow or severe weather is forecast, the number of calls can quadruple. Officials of the federal government -- the area's primary industry -- suspect that many of those calls are made from the office by some of the 350,000 federal employes here.

The GSA phone system also blocks calls to the 976 prefix, used primarily for prerecorded messages such as Dial-a-Horoscope, which cost from 20 cents to $1 each. Blocking them will save an estimated $40,000 a year, GSA predicts. Special surcharges started being placed on the dial-a-number calls in March as an outgrowth of the national deregulation of telephone services.

There are about 28 dial-a-numbers (ranging from Dial-a-Saint to Dial-an-Atheist) listed in the D.C. telephone white pages. Because some of them have the same three-number prefix as other businesses, they are not affected by GSA's electronic blocking.

The telephone blockage system follows on the heels of a government crackdown on long-distance telephone abuse begun earlier this year, when federal agencies began checking computerized lists to see if numbers called from government offices were business related.

Officials said conversations would not be monitored, and said the checks were not being made to intimidate workers who might be calling congressional offices or the news media.

In some agencies the number of long-distance calls jumps around holidays, or just before special occasions such as Mother's Day, the officials noted.

Where agencies use a computer-run Centrex system of the sort used by many private businesses, lists of telephone numbers dialed and the numbers from which the calls were placed are mailed to bill payers every month.

Last month a Pentagon unit assigned a team to check long-distance calls made over a two-month period. The auditors, in turn, ran up a telephone bill of several thousand dollars because they had to dial each long-distance number to check out who was being called, an employe in the office said.

Employes were subsequently charged for personal calls identified in the audit. The biggest bill was run up by a Pentagon supervisor who had made calls to a relative in Ocean City, Md.

The electronic telephone blockage started in New York and Chicago also prohibits calls to so-called Dial-a-Porn or "adult-oriented" telephone numbers. That latter prohibition is being challenged in courts as a violation of freedom of speech.