Buy a cheap phone at Charles de Gaulle?

Any reason you must buy it at the airport? Relay Stores seem to have an exclusive for phones and SIM cards at CDG. Their selection and prices may leave a lot to be desired. I think you will have more options in Paris or wherever your destination might be. Go to the Travel Tips section of this site, click on "Phones and Tech Tips" for a wealth of information. You might also want to consider buying an unlocked GSM phone before you leave (just google or check eBay) and then buy a SIM card in France.

I agree with Philip about buying an unlocked cell phone before you leave. You may have an old phone at home you can use. If it is AT&T or T-Mobile and a few years old, call the company and tell them you want to unlock it. Or buy a cheap unlocked phone at Ebay or Amazon. When you get to CDG, go to a Relay convenience store and ask for a Lebara SIM card--they have great rates and a website in English. Pop it into your unlocked phone and you're ready to go.

Please note that not "any old phone" will work in Europe, even if unlocked. The phone must be not only GSM (T-Mobile or AT&T in the US), it must also have the right frequencies to work in Europe, which doesn't use the same GSM frequencies used in the US. The key frequency you need for voice service is 900MHZ in much of Europe, but old GSM flip phones used in the US may only do 850MHZ (plus a few higher frequencies).

I have two older T-Mobile flip phones, and neither of them worked by default in Europe. I was able to hack the older one to do 900MHZ instead of 850MHZ and used it a few times successfully. I turned on the other one (which could not be hacked) once for fun when I had it with me in Belgium, assuming it wouldn't work, and indeed, it did not pick up anything.

If the old phone is a "quad band" or "world" phone it will probably work. The best way to be sure is to google the exact model number and find its specs. If it has 850MHZ but not 900MHZ, it probably won't work in Europe (not everywhere, anyway).

Most smart phones do seem to have the 900MHZ GSM frequency, though.

I'll second the recommendations to buy a phone ahead of time. Like you, I use a cheap prepaid phone at home; a few years ago before leaving for France I bought a quad band unlocked phone through ebay for about $35.

You can get SIM cards at the stores at the airport, however, it looks like they don't sell phones anymore unless someone else knows different? A post I saw from 2014 says they sell SIM cards, but no phones.

It might be cheaper and easier to set up a post-paid account and a quad-band world phone here in the USA to use over there with roaming calls. As long as you are only going to make a few short calls, even with the more expensive phone companies AT&T and Verizon, roaming should be no more than $1-$2 a minute.

The best deal going right now for European travelers is T-Mobile. They sell certified preowned iPhone 5S for $340 and the 1gb plan for $50/month. That plan includes unlimited data and text messages in Europe and calls while you're in Europe are $.20 a minute to other phones in Europe or back to USA. Oddly, calling Europe from the USA requires a "Stateside International" plan.