I went through the exact same thing. I was an hourly employee at my non-union hospital for 31 years. My schedule was M-F from 0600-1430. However, because I was the only maintenance engineer person at an outside property (Surgery Center) I was the only one who knew that building and expected to be on call 24/7 (without any extra pay). I was called in many times day and night on my days off for one thing or another. Same as you, they would ask me to take comp time off during the week and not at the OT rate. If I was called in for 4 hours on a Sun. they wanted me to take off 4 hours instead of the time and a half 6 hours I was entitled to during my regular week. Their reasoning is that because the maint. dept. does not do direct patient care is a cost to the hospital and they try to stay within a certain budget. When they have to pay OT they have to account to administration as to why they went over budget.
They knew I was a nice guy and would always do whatever needed to accommodate the hospital. If I requested the OT they would pay it but then I was looked at as not being a team player and my annual evaluation could or would have several negative remarks which could and would affect or delay any raises or promotions.
I was lucky that I had the chance to retire 19 months ago. I still keep in contact with my immediate supervisor who I've known for the entire 31 years I was there. He tells me its only got worse since I left. He wishes he could retire but he's not of retirement age listed in the employee handbook. The handbook is another thing administration has re-written many times when an employee found a loophole or they reduced or eliminated benefits.
When your position changed a while ago you should have contacted a lawyer to draft / create a specific job description of what is required of you such as call in pay, benefits, OT rates, pay rate, future raises etc.
I'm afraid you are between a rock and a hard place and that the bottom line for you is put up with their requests, leave for another job, or retire.