[andy]

I spoke to O2 customer services, and there might also be:

Austria: T-mobile and telering
Switzerland: Swisscom and Sunrise
Spain: Movistar and Vodafone, but not Orange

but I'm not that confident in what she was saying, after she repeated the suggestion to use a credit card, I asked her if she herself had a Dutch or German credit card, and she asked what that had to do with it.

I think you can get German top-ups on a site that people have mentioned on here, but without checking I don't remember the name

But you might as well wait until you get here, and in between just use it a slight amount every 180 days, as the requirement to stay valid is usage not top-ups