Thank you for your answer.
I often use my phone to call abroad, so I guess the problem comes from the mobile I am trying to call. The mobile I try to call receives sms but when I try to call from my home phone it says that the number does not exist. ("le numéro que vous demandez n'est pas attribué") If I try with my mobile, it refuses to call and I can read "service unavailable" ("service non disponible") on my phone.
I assumed it was because the person has no more credit, but I may be completely wrong.