Practically speaking, I already do: I told them not to contact me on my cell if they aren’t going to give me a work phone or reimburse me for the cost of a plan then I will not be using my personal phone for work purposes. They can reach me during business hours on my desktop phone, the work messaging system, or a ticket. If anything happens outside of those hours and they want me to do something about it, they better pony up for a plan, overtime hours, or flex hours.
Firing me seems highly unlikely because they agreed to these terms, I am very good at what I do, and they love me. Actual legal protections sound like something everyone should have though, if you’re on call you damn well better get paid for it or it’s wage theft pure and simple.