In the United States, Federal Communications Commission rules require users of selective calling to monitor the channel, "i.e.", switch to carrier squelch, before transmitting. In other words, the user must monitor (listen) to make sure the channel is not in use by someone on another selective calling code before transmitting. To enforce this rule, base stations often have a monitor switch on the microphone. The push-to-talk button is split into two segments. One segment turns the selective calling off. The other segment of the button transmits. A mechanical interlock prevents the transmit button from being pressed until the monitor button is down. This is called, "compulsory monitor before transmit". In mobile radios, microphones are stored in a hang-up box. When the microphone is pulled out of the hang-up, the radio reverts to carrier squelch and the selective calling feature is disabled. The user automatically monitors—verifies no one else is using the channel—by pulling the microphone out of the hang-up box. Hand-held radios sometimes have LED indicators that show when the channel is in use.