Duplexer plays the key role to allow two way radios to operate in a full duplex manner. It mean the transmitter and receiver can operate simultaneously as opposed to the old time walky-talkies -> ‘push-to-talk’ manner which operate in non-duplex mode. It is because the transmit frequency and receive frequency are different.
Duplexer is a device which allows a mobile phone to operate like a wired telephone, with either or both people speaking at any time without using a microphone switch to enable the radio transmitters.