What do you call that sound that plays before and after someone speaks in an intercom?

i don't know about intercoms, but for 2 way radio transmissions, a beep tone is inserted at the beginning and end of transmissions. the beginning is an alert tone to open squelch circuits in the relay paths so that the following voice message is ped. the end tone relieves the talker of having to say "i am done talking, over to you, i am now listening" at the end of each transmission. CBers some time use the tones, called "Roger Beep" in their lingo, but since their transmissions are not relayed, it is just a gimmick and not needed.