You can send a status report every few hours (I usually do once every day or after sinking a ship). If you are tasked to "conduct anti shipping operations" at a certain area and you have sunk a certain number of tons, you just radio your status to your home base and get a new mission (if you still have torpedoes). The same goes for special missions. After dropping your troops, agents or cargo you have to send a status message to get a new mission.
The contact report can be send after an enemy task force has been spotted. You will then get a reply to attack that task force or resume to your original mission.
