Well I just found the function that's responsible for the game radio messages

So one down, many to go!
Here's an interesting tid bit of info: while decoding this function call it called out to another function at the start that compared the sub's true depth to -20. The result of that determined whether you received the radio message or not. So 20m depth is the cutoff point where the game says you cannot receive radio messages
This was an important find (function responsible for radio messages) because I need this for some upcoming ideas
EDIT: it appears that if the message is critical then depth is ignored