"Frequently they don't get called during their week at all and it's free money."
That's, I think, is a wrong perspective, when people are on call, they have to be somewhere near their computer/internet connection and be ready to work (so it is not just you can go to a party and if call happens do some quick fix in a toilet).
On-calls cannot do what they want with their time, so they don't get money for free.
Yeah this is true although our compensation for their time is pretty generous. As a team we are pretty flexible too and if somebody has an event they want to go to they can always find someone else on the team to cover or swap with them for the night.
That's, I think, is a wrong perspective, when people are on call, they have to be somewhere near their computer/internet connection and be ready to work (so it is not just you can go to a party and if call happens do some quick fix in a toilet).
On-calls cannot do what they want with their time, so they don't get money for free.