Aldi's MedionMobil costs 0,03€ per minute for calls within the network, but even better: they have a 3,99€ 30 day flat rate package including all in-network calls ("Community flatrate"). They also have a 30 day German landline/in-network flatrate for 14,99€. The cards are valid for 12 months from first activation; reloads add 360 days to validity. Refills of 30€ or more at once add 720 days to validity.
http://www.medionmobile.de/index2.htm

As for phones, I'd buy Quad-Band unlocked phones before going. There is a recent thread in Travel Tech about unlocked phones.