Service techs are not emergency workers and give up their prsonal life the entire time on call. Most large companies will pay 4 to 5 regular hours along with time and a half for anytime out on a call. Sounds like your employer is dancing on the edge of violating wage and labor laws. If your total hours worked and on call 24/7 hours add up to less than minimum wage when divided into your yearly pay, then they are not being fair with you and could face charges. The problem is you would have to be the whistle blower and that could end your career.
Find another place to work.