A black book containing confidential telephone numbers of the command post outside President Reagan's hospital room and notes about calls to the president -- one of them threatening -- apparently was left by a Secret Service agent in a Fairfax County phone booth last week.

The book was found in a pay booth outside a bar on Route 1 south of Alexandria Friday night by an aide to Rep. Standord E. Parris (R-Va.).

Parris turned the book over to Secret Service director H. Stuart Knight yesterday afternoon. Parris also gave a letter to Knight in which the congressman said he was "dismayed and somewhat angered" that someone ". . . could be so lax in his duties as to leave a book with sensitive and confidential phone numbers lying in a phone booth."

Parris asked Knight to tell him "the name of the agent who misplaced this book and his explanation of how it was lost."

Secret Service spokesman Richard Hartwig said yesterday, "It looks like [the book] belongs to an agent, but on inspection we could find no classified information per se."

The notes apparently were recorded at the Secret Service command center at George Washington University Hospital on the day Reagan was shot. A hospital employe said yesterday that Secret Service agents "had extra telephones installed and also took over some of our phones" after the president arrived there last Monday afternoon.

Notes on one page said a white male with a Southern accent called at 3:38 p.m. and said "there's going to be another threat on the president's life."

A caller from New York asked agents to "have Haig call me" and left a call-back number. A person contacted yesterday at that number, which is listed to a bicycle manufacturing plant on Long Island, said there was no one there by the name recorded in the book and that no one had attempted to call the secretary of State.

The name of Tom Getman, an employe of Sen. Mark Hatfield (R-Ore.), was recorded in the book at 4:52 p.m., which Getman said yesterday was about the time he called the hospital last Monday. He said he spoke to a Secret Service agent and explained that Hatfield was attempting to reach Senate chaplain Richard Halvorsen, who had come to the hospital to be with the president's family. The notebook contained both Getman's number and Hatfield's private line.

The first page referred to an appearance by Vice President Bush at the National Press Club. A spokeswoman for Bush said yesterday he last spoke there at noon on March 20 to the National Newspaper Publishers Association.

Another page contained the number of the Special Operations Division of the Metropolitan Police Department, along with an arrival time, parking bans and secure rooms at an unidentified location.

Supervisors of the Chesapeake and Potomac Telephone Co.'s "ESS" division were named, next to a notation that one "number is blanketed." A C & P spokesman said yesterday that ESS stands for "electronic switching system." He said ESS has nothing to do with surveillance and that "blanketed" did not refer to a tapped phone. The C & P spokesman suggested the information was related to the installation of special lines at the hospital.

Not all of the messages were menacing or mysterious.

An elderly woman whose name and number were recorded at 4:39 p.m. said yesterday that she had called the hospital before the president underwent surgery. She wanted "to let him know that I knew a little girl who was accidentally shot by her grandfather more than 50 years ago," the woman said. "The bullet was never removed, because it was too close to her heart, yet the little girl grew up and had a normal life."