[ronwi]

I see that a number of the German carriers have prepaid flat rate plans of EUR 15 - EUR 20 per month, allowing unlimited calls to German landlines and on-network mobile calls.

Betamax has local rate numbers in Berlin and Muenchen that allow you to call over the Betamax network at their regular rates (including free minutes.) If I signed up for the flat mobile rate plan, would calls to the Betamax number be covered in the flat rate plan?

The number for Berlin is given on the Poivy website as 03031190303.