Quindar tones

It was a means of in-band signaling to simulate the action of the push-to-talk and release-to-listen (often referred to as PTT) button commonly found on two-way radio systems and walkie-talkies.

However, even though the tones are no longer necessary with modern digital communication systems, they have been retained both for their historic aspect, and because flight controllers say they appreciate the audible cue of an imminent transmission.

Since dedicated phone lines were a very expensive measure at the time, NASA chose the use of tones to reduce the operating cost of the network.

[1] The same system was used in Project Gemini and was still in use with half duplex UHF Space Shuttle communications for transmitter radio frequency keying.

When finished speaking, the CapCom would release the PTT, which would send the outro tone, and the astronauts would respond to Mission Control.

Additionally, separate radio frequencies[3] allowed both Houston and the astronauts to talk simultaneously if they wished and thereby made a courtesy tone as a way to minimize the possibility of both of them speaking at the same time unnecessary.