ALTA, apparently a US translation company, reports in its blog on How not to flirt in German. I can see more than one reason why not.
Kann ich dir meine Handynummer geben, gerade falls Hälle über einfriert?
Can I give you my cell number in case Hell freezes over?
LATER NOTE: The texts have been changed – better German, but the humour is missing. They can be found here too.
My original German header was a bit ruder than even I intended. I tend not to use my middle initial A. but it’s there!