[speed150mph]

As the others have said, there is no way for a submarine to establish long range communications from deep depths. In the Cold War, and in the russian navy still, a submarine can be called to the surface by an ELF transmission, but the frequency is so low that it can take a minute or more to even send a single letter in a message, but can penetrate the water.

For a submarine to transmit or recieve longer messages like updates or orders, they need to come to periscope depth and raise the mast or stream the floating wire depending on situation. The mast can be picked up on radar, but you can also put up your other masts and periscope and take a peek and listen to other activity around you. The floating wire is less detectable on radar, so would be used in times where possibility of detection was high.


As for why a submarine would transmit, the main reason would be just to check in at home, post a position report and let their bosses know that everything's fine. If a sub is out of contact for too long the are declared missing and a search is conducted...

Other reasons include contact reports, intelligence reports, acknowledgement of orders, damage reports. Pretty much anything that their squadron CO would want to know immediately. Obviously in times of war or during covert ops, these would only be done when able, a submarine attacking a carrier group won't tell the boss he's about to shoot and give himself away.