Complex sentence: Probationary firefighters can be full operating firefighters and respond out to a call and go into the buildings, as is the case for paid departments, or they can be firefighters who do not perform interior operations, as is the case with most volunteer departments.
Split into simpler sentences: Probationary firefighters can be full operating firefighters, responding to calls and entering buildings, as is the often case in paid departments. Alternatively, they can be firefighters who do not perform interior operations, as is the case with most volunteer departments.