Company is implementing mandatory, uncompensated on-call

Context: Salaried employee living in California, working for a fully-remote software startup.

After two years on-call is being implemented. It’s unpaid, and mandatory. With current rotations I’m looking at 10 weeks per year. On-call was not previously required, nor does it appear in my employment contract.

I’ve done some reading and it appears that as long as there aren’t overt restrictions to movement then unpaid uncall is fine.

However, they’re expecting 10-15 minute response times and you always being in a location with internet service.

Additionally, these text alerts are expected to be setup on our personal devices and phone plans. The company does not contribute towards these costs, nor do they issue work phones.

Does that constitute as overly restrictive? And if so, do I have a case?