SFR's support pages only show how you can make your mailbox not recording voice messages but to announce your unavailability instead:
Les options du répondeur Mobile
Dial 123, press 3 for "options" and then 6 to switch to "mode répondeur-enregistreur" (non-recording mode).
Unfortunatley I couldn't find out if callers will still be charged for this announcement or not. Once you find out please report back.