Yeah, a mandatory work phone (where the employer can define requirements) should be purchased and funded by the employer.

I must assume they mean no use it for their business that they own. Otherwise I'll just be sad.

Work profiles exist for this reason. A completely separate environment that is isolated from the rest of the phone and can be turned on and off on demand.
Some manufacturers, like Samsung, make it difficult to access, but it's there.

And the obvious annoyance of having two phones