How long are you going to be in Paris for and do you have a permanent address or a hotel address? I would highly recommend B&You. I lived in France for four months as got their EUR19.90 package which had free calls to mobiles in the US, Canada, and China (inc. HK
and Macau), free calls landlines in 55 countries, and tons of data included, all without a contract. You can book it online at www.b-and-you.com
and they will send you the SIM card to your home or hotel within 3 days.
I've also used it to roam in Italy and Morocco and not only does it work great, the roaming rates were relatively cheap.