I wouldn't, and it is decided by new German telecom regulation that went into force last year. The card I'm using in Portugal no longer works when I visit Germany, because it's a pre-paid card.
Thanks for explaining. I didn't have a pre paid card for as long as I can remember. Strabge so, my son's prepaid card works just fine in the EU outside of Germany...
Edit: Just realized, that's not a prepaid card anymore, so forget everything I said...