Are employers required to notify a terminated employee by letter, email or phone call?

I went on approved medical leave 3/10/15. The company approved the leave until 9/13/15. When my physician said I couldn't return to work yet I followed procedure and called the Leave Dept. I advised them that I did not know my return date. I was being paid through short term disability (company paid). When I was unable to return on 9/14/15, the insurance company UNUM, began the process to switch me to long term disability (employee paid). I called the company for an unrelated reason last Wednesday and was told by a rep that I was terminated 9/14/15.