Ex-presidents like to keep in touch. During fiscal 1984, the nation's three former chief executives ran up telephone bills of more than $72,000 and spent another $28,000 for postage, the General Accounting Office said yesterday.

The GAO reported to the Senate Appropriations Committee that the government paid nearly $26 million for services associated with ex-presidents and the presidential library program.

Gerald R. Ford had the largest phone bill at $30,281. Jimmy Carter's bill was $21,589 and Richard M. Nixon's was $20,704. Each former president spent more than $9,000 for postage. In addition, the government paid $2,600 for postage for Lady Bird Johnson, widow of Lyndon B. Johnson.

One declining cost is Secret Service protection, which reached $11,750,000 three years ago and then dropped to $10,547,659 during the 1984 fiscal year.

During that period, the number of people entitled to protection dropped because of the death of Bess Truman and the fact that Amy Carter turned 16.

Nixon notified the government last spring that he no longer wishes protection.

Each former president receives a pension of $82,438 and more than $100,000 for staff. Mrs. Johnson receives a $20,000 annual pension and no staff allowance.

The government paid $21,075 for Ford's travel last year, $11,470 in travel expenses for Nixon and $1,187 for Carter.

The amounts spent for pensions, staff, supplies and travel by each of the ex-presidents was $363,504 by Carter; $360,807 by Ford and $327,066 by Nixon.