I don't know anyone who is paid to be on call at two different employers simultaneously.

neither do i. all of the on-call folks I know must respond to pages; they don't have the option to pass on them.

that's just the existing on-call model. it's quite easy to imagine that a service overprovisions to account for people not being required to accept a "page". that overprovisioning specifically is so that people can hold multiple on-call type jobs simultaneously.

But I think if Uber and Lyft are required to pay their employees an hourly rate they would then require them to respond to every page for their scheduled shift.

They don't now? I'd heard both would boot you from their network if you turn down too many rides.

Back when I had to work be on call there was a minimum number of hours, so if I got a call at 2am from a client and I fixed the problem in 15 minutes I would still bill both them and the company for 2 hours. Not sure if there's a law mandating this or it was just the company policy.

On call and showing up for a work request are both mandated by law in CA. That's where a lot of issues people are overlooking lay with Uber/Lyft having employees. In their current model they would have to pay drivers just for logging into the app. If the driver did one customer drive and it took 10 minutes, they would have to pay the driver minimum 2 hours.