Call Waiting (CW) is a service feature in digital telecommunications networks. With this service feature, an incoming call request can be signalled acoustically by the call waiting tone or optically during an existing call. This service feature allows subscribers to remain reachable by others even during a call.
The call waiting service feature is also offered for multi-device connections and is adopted by PBXs. The called party recognizes the caller's number in the display and can react immediately to the call waiting by interrupting the existing call and letting it rest in order to accept the second call. This switching back and forth between the previous caller and the waiting caller is called toggling. If the waiting party is actively connected to the existing call, this is called a three-party conference.tone, while the called party hears the call waiting tone. Like most ring tones, this tone has a frequency of 425 Hz. However, it differs from the busy tone in that the interval between the tone signal and the pause signal is shorter (200 ms). If the subscriber does not comply with the connection request of the call-waiting party within 30 seconds, a busy tone sounds instead of the ringing tone.