Renting a cell phone

In France you will see cell phone stores all over the place. Just buy a pay as you go one. Many places offer dirt cheap phones, some are free, and you just pay for the prepaid airtime (usually 20-40 euros).

Many companies, like here in the US, offer free mobile to mobile, or reduced rate for calling within the network.

I have a follow up question: We're going to Germany for a few days and then on to London. If I buy a pay as you go phone in Germany, will the same phone work, but with a different SIM card, in the UK?

Unless things have changed in the last year or so you will need a new sim card for each country. Most low-end phones don't offer you the option of switching cards (they are low end for that reason) and are locked into a particular company.

Is you U.S. phone tri-band can it be unlocked to work over seas? You might be better off that route, or looking to see what it would cost you to upgrade your US phone to one with a tri-band.

When I was in London I got my Virgin Mobile with a removable sim. When I got to Germany (Munich) I could change the sim or use the same sim. The Virgin Mobile greeted me "We hope you're enjoying your holiday" and knew I wasn't in the UK.

I didn't bother finding a new SIM chip because I gave my UK # to my family... and the cost to call outside the UK wasn't that expensive.

I think you should have no problem using a phone from Germany in the UK, France or whereever.

First, I loaned Monica my two phones so she is OK. I showed her the site with info on France's national carriers and their pre-paid rates so she can pick one.
I strongly suggest people read my Technology posting on finding cell phones for use in Europe. It tells you not only how to tell if the phone you have or are considering buying or borrowing will work there has information on locking and a site that tells you all the local carriers and their prepaid rates for countries around the world.
I am also willing to answer questions about these issues.