Not so sure about this one
Work phones are partly for making calls but more importantly for sending and receiving email. At my office, when you connect your mobile phone to exchange (or Office 365 or whatever the service is called now), it demands remote admin rights over your phone for the owner of the exchange server. They can remotely factory reset it if they like.
That's fine for a work device and you can choose to do it on your own tablet or phone too if you want. But your employer demanding admin rights over your personal phone as the only option for work calls? Not for me.