After you send a message or location, the bivy stick modem will stay on for 15 minutes to send the message and also listen for incoming messages. Every 15 minutes the modem will wake up and do a mailbox check. It will try for 3 minutes and if it doesn't get a response, it will quit trying. If you're expecting a message and need it urgently then you can do a mailbox check to get it quicker. 1 credit will get you 6 mailbox checks.