If you are in France and want to call someone living in France, dial a ten-digit phone number, always starting with '0'. If your want to call someone abroad, dial '00' + country code + phone number.
If you want to use you current GSM-compatible phone in France, you have two options:
Keep your current service provider from your home country: Check with your provider that you can use the phone in France (roaming) - it is unlikely you will not be able to, but you may have to specifically ask your provider to enable your account to do this.
This is not a cheap solution as you pay for all of the calls you receive at international rates (for the leg of the call between your home country and France), in addition to being on expensive tariffs for calls from France. You do receive the benefit that calls you make to French numbers when in France are not international calls, but billed as national.
Buy a new SIM card: This will give you a French phone number, so you can call on normal mobile rates and not pay anything for receiving calls. You need to check whether your phone is unlocked (some phones are specific to only one network). Even if your phone is locked, it is easy to unlock it (ask in a specialist telecom shop, unlocking your phone might cost €20-30).
There are three main providers in France, each with its own coverage and sales network:
Orange ( www.orange.fr): Subsidiary of France Telecom, which acquired Orange in 2001 and renamed its own network in France from Itineris to Orange shortly afterwards. Website is mostly in French, but there is a small section with some basic information in English (go to 'Welcome to France' in the navigation bar).
SFR ( www.sfr.fr): This is today a joint subsidiary of Cegetel and Vodafone. SFR has a good website for foreigners coming to France at www.sfr.com. The website is available in 6 languages (French, English, German, Spanish, Dutch and Italian) and gives a brief introduction on using mobile phones in France and the main SFR services.
Bouygues Telecom ( www.bouyguestelecom.fr): The 'third' operator - used to have more limited coverage that the other two, but this today only affects some limited areas.