be in touch with. Be in communication or contact (with), as in Be sure to be in touch once you've arrived, or Our representative is really in touch with her constituents. A related idiom is
get in touch, meaning “initiate contact,” as in We tried to get in touch with you but you were out of town, and
keep or stay in touch, meaning “remain in communication or contact,” as in With Jim stationed in Korea, it was hard to keep in touch, or Do stay in touch with us. This idiom transfers physical touch to communication.
[ Late 1800s