Also known as ‘buy the company a phone and pay for its service.’ In order to enjoy the expectation of 24x7 availability.
And of course you still need a second phone for your personal stuff, unless you enjoy having some random in the IT department listening in on all your phone calls and sniffing your bank account password.
Mind you, I don’t see anything wrong with a BYOD policy if it’s disclosed before you decide to take the job and you’re compensated accordingly. There are many situations and industries where, for instance, a worker is expected to bring his own tools - and many trades, the workers prefer this, since they can get the hammer whose grip fits their hands, and so on.
In many ways, the real issue is that companies want to nickel-and-dime at both ends. They get away with lying about their prices by attaching hidden fees that are really part of the cost of doing business (ISP’s, phone companies and airlines are the worst at this, but it’s catching on), and with lying about the compensation that they offer employees by attaching hidden conditions (you have to provide your own phone; unpaid overtime and on-call time is exepcted; you can take your vacation time only if you’ve amassed sufficient billable hours; etc.) Moreover, at both ends of the deal, they often get away with adding these conditions of contract on customers (who sometimes lack the recourse even of cancelling service) and employees (often with no consideration other than continued employment).