I wouldn't put too much hope into this one at the moment. The last teltarif article states that the SMS/call prices should be added soon. Walmart states in the article that they'll have 88 of their German superstores equipped with MangoMobile call shops by April.
http://www.teltarif.de/arch/2006/kw03/s20173.html
For the moment, it seems this MVNO is still in its infancy so let's give it some time.