First of all, I do not understand why you can not send the details by e-mail. :o)

Whatever - the 800 number sounds like a national service number that does not work for international calls. Therefore you probably just forget about that number.

541 is the area code ? Would be (0)541 in total and sounds like the area of Osnabrück.

Tell them where are you calling from and they should give you the exact number.

That´s all I can say right now. In case you tell us the car hire company name and address we could be able to organize you the right number.

Martin :-)