Nick, check the archives here about this topic. iI's been disucssed several times.
BMW Assist uses a module that accesses over the verizon network. But it's a pay service similar to OnStar. Your call can't be completed because you do't have an account. And even if you did have an account, it's an emergency-based access, and not intended for regualr cell use.
In order to get bluetooth into the car, you need to install a bluetooth module that will pair up with your wife's cell phone. It's an easy thing to do, albeit an expensive one, by purchasing an aftermarket retrofit kit, like the one being solde at
www.bimmernav.com. The kit includes a ULF module that is installed in place of your existing BMW Assist module, a pairing button (or eject box to hold the phone, if desired, ) a bluetooth antenna, and a microphone, if needed. Everything plugs into existing wiring and sockets, and it can be installed by a knowledgable person in about thirty minutes. Instructions are pretty easy to follow. I've done it three times, (2003 E39, 2004 E46, and 2003 E53), and it works like a charm.
Check that website, find your car, and pick what you want. I found them to be very helpful with information if you have questions.
Good luck,
Dave