Based on what I have read in sub books such as "Clear the Bridge" American subs never used the radio to talk with other boats. It was always done face to face based on prearranged times and locations. Radio traffic would be a dead give away to the Japs as to the location and number of boats in a particular area.
Another way they communicated was using their radar to alert other US boats of their presence, especially when approaching a convoy or as a direction finder.
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