If it's in the contract you sign, then it's already priced in. I don't see much value in specifically outlining which part is base and which is for oncall, if oncall is mandatory.
And sure, it is priced in because the companies can get away with it. This doesn't mean it is right or fair as the price is always going to be in the company's favor and rarely fair to the employee. For example: My brother worked for a US railroad. He didn't have a set schedule. Instead, he got 10 hours rest after a shift and then he was on call. They only closed down on Christmas and New Year's. You were expected to do this on-call work perpetually. The money was good for the area as were the benefits. They advertised in depressed areas without much opportunity, so it made it easier to prey on folks that will accept the poor treatment. I'm pretty sure having to pay folks for each of those hours would change the behavior of the company in the employee's favor.
So yeah, even one hour of 'on-call' should be paid extra, outside of your basepay. Even for companies and industries that aren't being actively evil so that it doesn't happen in the future.
My contract says that if on-calls are needed, I might have to be in the rotation. This clause increases my pay rate even if I'm not on-call.
If however I am actually on-call, I am paid more. And if the on-call rings, I'm again paid more on top of the on-call period. And as the French law mandates 11 consecutive hours of rest, if the on-call rings in the middle of the night, I'll usually come to work later the day after.
If you don't have advantages for being on-call, you're the one being taken advantage of.
Usually those hours are harder to predict compared to working hours so it is much easier to pay for them outside of typical monthly salary.