Marine VHF mostly uses half-duplex non-relayed transmission. Ship to ship communication is over a single radio frequency, while ship to shore uses two frequencies, but typically only one of the parties can transmit at a time. (Not to be confused with simplex communication, for example radio broadcasting, where one party transmits always transmits.) The transceiver is normally in receive mode; to transmit the user presses a "push to talk" button on the set or microphone which turns the transmitter on and the receiver off. Some channels, however, are "duplex" transmission channels where communication can take place in both directions simultaneously when the equipment on both ends allow it. Each full-duplex channel has two frequency assignments. Duplex channels can be used to place calls over the public telephone network for a fee via a marine operator. When full-duplex is used, the call is similar to one using a mobile phone or landline. When half-duplex is used, voice is only carried one way at a time and the party on the boat must press the transmit button only when speaking. This facility is still available in some areas, though its use has largely died out with the advent of mobile and satellite phones. Marine VHF radios can also receive weather radio broadcasts, where they are available.