How much does "on call" pay?

I'm salaried, I don't get extra the holidays I'm oncall. I would if I were hourly.
When we moved from hourly to salaried about 8 years ago, we got a pay increase that was supposed to make the difference of the lost OT wages. It did not.

The Doctors on call here on the Nullarbor don't get paid anything extra. And they get paid way less then Doctors in the city.

Yeah or extra for when you do. You're getting screwed.

You can choose to not be on call if you're not getting paid can't you?

It's restaurant work

Depends on the job. On call physician or on call event waitstaff. I've done the latter and that was 10 bucks an hour 10 years ago. If the nature of the job means that it is on call the employer might even view this as a perk for you in terms of "freedom" and pay you little.
If it is a highly skilled profession and most people who do that job have set hours then you would get paid more than the average.