If your T-Mobile phone is unlocked, you might want to get a Bouygues Carte Nomad SIM. On their website it's €15 (includes €4 of credit), but I'm guessing you could find the same deal in their boutiques or maybe even in general mobile phone stores. You can then buy recharges in various amounts to add to your talk time, and your phone number will be valid for eight months from the date of the last recharge. If your trip is short, use the "Avantage Flash" option which will give you twice as many minutes for half the usual duration (for example, 40 minutes to be used in 15 days instead of 20 minutes/1 month).