Well thanks to a comment on the forum an alternative may be available from your local supermarket.
The E.Leclerc Mobile service lets you top up your phone credit whenever you want and retain the value of it for a full 12 months.
Even if you top up the phone towards the end of this 12 month period you still hold on to the earlier amount and can continue on using the phone.
The only cost involved is a €1.50 a month service charge, but you can also hold on to an existing mobile phone number as well as continue using your current phone.
If you don't have a mobile phone then you can buy a basic model as part of a package, with the option to purchase phone credits to the value of 5, 10, 20, 30, 50 and 100 euros.
The website is easy to use but if you want to print off some documents to get a better understanding there is a download area that may prove handy.
Have you used the E.Leclerc mobile phone service? Or if you know of an alternative please feel free to leave a comment below.