Today I was in the same situation - in Greece!
My SFR was in the "hibernation"-status (after expiry before switch off).
Calling to 950 worked, when pressing "1" (check credit) I got told that I do not have enough credit (in french) to check my credit (!). After dialing again and pressing "2" (top up) I got the message that ther number is incorrect (!).
When dialing the 952 I got the same message: The number is incorrect.
After waiting for a while (around 15 minutes) surprisingly it worked: I could dial the 952 and top up the SFR sim with a 5 € Vodafone Greece voucher (3 € credit).
The best thing: The validity for the 3 € is one month.
So it seems the system in France needs a little time to work properly.....