A while ago after seeing MIB's most-cool phone setup I decided to order the same. I ordered the Nokia 6310i (
http://www.nokiausa.com/phones/6310i), used it to make a few calls (literally), but never installed it as work pays for my phone (and I am on evil Verizon).
I paid $200 for the phone (is a GSM phone), and another $50 for the snap-in adapter. It's a nice phone, and it needs a good home. The phone fully integrates with the M5, and can still be found for sale for $165.