@Roberts_1132729,
You don't say what add-on plan you have, which would have been useful to know, but am going to a presume it is an International calls, texts and data add-on.
If you have this plan you should be able to phone a French phone. Some things to consider perhaps;
Are you using the right International dialling code?
Is the number itself being put in correctly?
Could the person who uses the phone have changed their number?
There may be other reasons you are not getting through
