what does "depressed his receiver"mean ? make Johnson depressed?
"Congressman Albert Thomas thinks I should take the oath here," said Johnson, citing support. There was no answer, and he pressed on. "A lot of other people feel the same way." The phone by the pool remained silent. Kennedy did not dissent; he said nothing. Changing to another tack, Johnson again referred to the plot, and then he requested information. According to Youngblood he asked "questions about who, when, and how he should take the Presidential oath." Kennedy heard, "Who could swear me in?"

"I’ll be glad to find out and call you back," he answered.

He depressed his receiver and asked the operator for Nick Katzenbach. It was 3 P.M. in Washington, according to Katzenbach's secretary's log, when, for the first time since the assassination, Robert Kennedy talked to his Deputy Attorney General. According to Katzenbach, Kennedys voice was "matter-of-fact, flat." He told Nick, "Lyndon wants to be sworn in in Texas and wants to know who can administer the oath."

Question

what does "depressed his receiver"mean ? make Johnson depressed?
"Congressman Albert Thomas thinks I should take the oath here," said Johnson, citing support. There was no answer, and he pressed on. "A lot of other people feel the same way." The phone by the pool remained silent. Kennedy did not dissent; he said nothing. Changing to another tack, Johnson again referred to the plot, and then he requested information. According to Youngblood he asked "questions about who, when, and how he should take the Presidential oath." Kennedy heard, "Who could swear me in?"

"I’ll be glad to find out and call you back," he answered.

He depressed his receiver and asked the operator for Nick Katzenbach. It was 3 P.M. in Washington, according to Katzenbach's secretary's log, when, for the first time since the assassination, Robert Kennedy talked to his Deputy Attorney General. According to Katzenbach, Kennedys voice was "matter-of-fact, flat." He told Nick, "Lyndon wants to be sworn in in Texas and wants to know who can administer the oath."

Answer

Rather than returning the handset to its receiver cradle, he pressed the hook switch in the telephone base with his free hand to disconnect the call.  And started another call by releasing the hook.  In 1963 it was probably a dial telephone, and so he dialed 0 (the maximum arc of the dial) to signal the operator.  But this detail is not important to the story, so it is left out.

Answer

Old fashioned phone, pressing the receiver cradle probably buzzed the operator in the reception/phone office, connected him to the phone operator.

Answer

He took (stopped pressing) the telephone receiver (handset).

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