I was 'on call' for almost 2 years with one company I worked for, and almost got fired because I slept in on a Saturday and didn't answer my phone for about 3 hours. After that, I called a meeting with my manager and HR exec. I told them if I was to be considered 'on call' at all hours of the day, 24/7/365.. then they needed to pay me for the hours I was expected to answer the phone.
I then laid out the math, considering 1/4th of my salary for 'on call' status outside of normal work, they would have to raise my salary to 75% greater than my current pay. I was told they would hire another person, and divide the 'on call' status across the group to make it 'fair'. After that, I received almost no calls outside of work(except on my 'on call' weekend). They figured spanning the 'on call' status to everyone on the team, was cheaper than any one of us getting that extra 75%.. and that's just at 1/4th of MY pay, and I was a mid-level dev at the time.
I got my free time back, and they were happy that I brought the fact that I was upset with the matter to them, rather than just leaving because I was mad at the situation.
On a side note: I used to work for a very well known in-house tech service.. on my week, once every 6 months, I was paid 1/4 time 8hrs a day for them to be able to call me at any hour, and have me answer. Even when I worked that day.