When you first got your phone, Cingular "locked" their SIM to the phone (in other words, the phone would only work with that specific SIM). Unlocking is nothing more than a programming change that lets the phone work with any SIM.

In your case, if you want a French number, you would simply buy a prepaid SIM in Paris, and you will have a local number with the cost advantages listed above. And since your RAZR is quad-band, there will not be any problems with European compatability.

On the other hand, if you Cingular account has international roaming enabled, you will be able to make and receive calls on your phone without having to do anything else - as soon as you power your phone on in Paris, it will register with whichever French cellular company that Cingular partners with, and you're good to go.

Keep in mind that people will have to call your US number - even if they are in Paris. The advantage to this approach is that there is no need to tell people a French number - if you expect only to get calls from the US and don't expect to make calls to French numbers, or to receive calls from French callers, this approach might better suit your needs.