As mentioned in the Dev updates thread, we now have an option to play with the simplified radio or a more realistic radio. "The new system has two modes: Simplified and advanced. The mode is set in the lobby settings before starting the game. In simplified mode, players type in messages using the keyboard, but unlike before, the message can be typed out in full, without having to pause between each letter while it is being sent. In advanced mode, the morse key functions like it does in real life. Players will have to learn morse code in order to use this system. The new system uses fixed receive channels for each U-boat. Channel 1 is used to broadcast to all U-boats."
When playing with the advanced radio (unchecking the simplified radio option), you will only know you are getting a message when you hear the incoming tones. You have to have a radioman stationed in the radio room, or one of your crewmen may hear the tones if they are close to the forward part of the control room.
I suggest sending a series of dot-dot-dot-dot for abut 30 secs, then pause before starting the radio transmission to give the other player time to get ready.
When you receive a message, you should send a short acknowledgement, perhaps,
CFM which seems to be a prosign for I ACKNOWLEDGE
https://en.wikipedia.org/wiki/Prosigns_for_Morse_code
If there are any experienced radio operators here, I would love to hear your ideas.