Ignore the email. If its really a problem for them, they'll come talk to you in person.

If they do, tell them (in a polite, appropriate way) that it's your phone, you pay the bill, and you'll set the voicemail greeting to anything you choose.

If they want you to be on call for work, and they want your voicemail greeting to conform to a script, then they should provide you with a phone.

Or just change your greeting to the "number you have dialed has been disconnected" recording.

Either way, if you get canned some bricks thru their windows should even things out.