During the very first season which was formerly called Good Morning, Miss Bliss (1987) (now referred to as "the junior high years"), we see Zack's father for the first time in the episode titled "Parents and Teachers" which aired on 12/28/1988, only his name was Peter Morris and he was divorced from Zack's mother. In this episode, his name has been changed to Derrick Morris and he appears to be married to Zack's mother.
Zack's father never hangs up his cellphone between calls, nevertheless, it manages to ring.