At our firm attorneys pay for their own cell phones. And if they want to access corporate data, then the firm retains the right to wipe the device clean should it become lost or stolen. That's a pretty boilerplate policy in most places, it protects both the firm and its clients.
If your firm is insisting on your using your cell phone for work (voice), then they really ought to fund that. You don't pay for the phone on your desk, do you?
And if your firm is insisting on your accessing email through your phone (data), doubly so. You don't pay for the computer on your desk, do you?
Personally, I refuse to get the Internet on my cell phone because I get the Internet at home and at work and survive just fine with those brief periods where I am not online. I find data plans to be utterly, completely ridiculous unless you are using them for work. In other words, the attorneys here genuinely need their data plan. The secretaries, who also have iPhones and Droids but have no access to the company network, simultaneously bitch about pay while handing Verizon an ungodly sum of money each year for their totally unnecessary toy.
Whoa, getting dizzy way up here on my soapbox, time for me to step down....