While on common phones two interlocutors can talk simultaneously and talk at the same time, in half-duplex transceivers the two interlocutors can only talk in turn, pressing the PTT key to talk and releasing it to listen.
Half-duplex mode is common on marine VHF, CB, PMR, VHF / UHF amateur radios etc.
For those who are not used to talking in half-duplex mode is often encountering communication problems, such as when the two interlocutors at the same time pressing the PTT switch talk at the same time and can not feel each other. For this reason, a roger beep was created to help mark the turn of the conversation.