Some monoth ago other FTers reported about problems with the US number (don't know to which country they route this calls) as this agents weren't able to deal with the problems. The german call center located in Rostock is really great if it comes to any problems with your rentals. They are pretty knowledgeable, fast and they know what they are doing ^ So I'd call the german number.

As pointed out before you may want to write an email. Contact information can be found on the sixt homepage. As I mentioned you shouldn't expect an answer but you can be quite sure that they will fix your problem within some days.

Cheers
totti