Yes, it has some substitutes based on the tone and urgency. Some of the most common alternatives are Call me when free, Ring me back or Please call back. In informal discussions, one may tell the other to hit him/her back or call him/her later. Longer sentences such as, Please call back when you are free are more professional. These options are flexible and you may change the way you communicate depending on the person you are messaging and the nature of your communication.
