In this respect the French telephone system is unique. French phones have a large flat plug or prise and a special socket to receive the plug. You can buy adapters from DIY and electrical outlets in France which will allow you to plug in a US style RJ11 plug into the French socket. The RJ11 is the type of small plug commonly found on modem leads. If you are connecting equipment which is fitted with UK (BT) plugs you need to first purchase another adapter which will accept your UK plug and adapt it to an RJ11 plug i.e. you will use two adapters connected together. This sounds convoluted but works fine in practice. It is worth purchasing some adapters before your trip to France as you may not be able to find the right items in local stores.
Alternatively, a number of people have found that they are able to remove the UK plug, strip back the wires and (largely through trial and error) connect a French plug (easily purchased at your local DIY store) to their phone and make it work.
Please note however that not all UK telephone equipment works on the French phone system so if all of the above options fail you may still need to purchase new phones in France.