E-Plus claims the number works from any country for free, where their prepaid cards roam, BUT unfortunatly only the recharge option works in those countries.
German info here:
http://www.eplus.de/dienste/8/8_2_1/8_2_1.asp
Alditalk itself only states that the number can be reached in Germany for free.
It also informs about deactivation after one year plus 2 months.
They will send an SMS once the 12 months without recharge have passed. Then you have two more months to recharge. If you don't, you can reach the service hotline under 01149-177-1157 to claim a refund for you credit.
http://www.medionmobile.de/guthabenkarte.php
Anyway, I would call 01149-177-1157 right now from any phone and ask how long the card is active. It may be that you need to provide some identification to get that info.
Hope that helps