If you have received a call from an unknown number there are several online ‘reverse directories’ which may help you trace who it was. However these may not help if the number is one that is internal to a company or if the user asked to be on a ‘liste rouge’ (ex-directory list).
For landlines try: www.pagesjaunes.fr/annuaireinverse (this is the Annuaire Inversé of the French yellow – and white - pages); annuaire-inverse.118000.fr (the same for the directory enquiries service 118 000) or infobel.com/fr/france
The following two may also identify mobile phone numbers: www.fasto.com and local.fr/annuaire-inverse