I think in that scenario,, you could separately open an account with a cheap provider that includes a free, cheap phone and dedicate its use to only work. So yes, pain in the ass worth extra steps, but not a requirement to use your own phone.

I think it's garage regardless, if they need you to have a phone, they should fully provide, but just pointing out that it's legal fuckery on their part as it's meant to confuse/scare people into thinking they don't have a choice.