Radio-telephony transmissions on any frequency, can be sent only one at a time. Simultaneous transmissions cause the messages to be partially or completely lost in a squeal of garbled noise. Other stations (the generic term for either a ground-station or an aircraft with a transmitter/receiver) within range, will recognise a simultaneous transmission, but may not hear the message or call sign - the transmitting stations may remain unaware that their messages have not been received, because operation of the press-to-transmit switch, blocks the receive function.
Source: Flight International