It's usually or 'keep radio silence for ...' or 'be more @#!=....'
I have not seen a response saying 'Do not attack the convoy, wait for reinforcements to come to your position' or stuff like that, because that's why I'm sending these messages.
Also important: there are 3 types of messages: Status Messages, Convoy Spot Messages, Contact Messages. Don't confuse them! In my experience, Status Messages do nothing but may get enemy planes to chase you; Contact Messages can get friendly planes come-in and attack your contact, but you need to be reasonably close to a friendly airport (you can check their ranges). Convoy Messages did nothing for me so far, I guess it will also cause friendly planes to come-in rather than other subs.
This makes me wonder: Are there any Wolfpack forces that can assist me in the game?