Chances are that any unused values or $ balance on that old Walmart or AT&T Worldwide Prepaid Calling Card had expired as it is usually valid for a limited time only & definitely not for more than 365 days, maybe 180 days once activated - so you will have to get a new one for prepaid calls back to Germany.

Places other than Duane Reade to find Prepaid Calling Card in Manhattan are at those newstands found everywhere not far from your hotel ... if you cannot find a DR nearby, any other drug stores/pharmacy chains will be fine - as they will carry a variety of prepaid cards, i.e. CVS, Walgreen or Rite Aid, plus, the "7-11" Convenience Stores.

For refilling my own prepaid mobile phone's Sim card accounts, I've been using an online seller - www . callingmart . com - used my credit card or paypal accounts to refill my AT&T and T-Mobile accounts, and I get a 5% to 10% off the card's face value, no fees or taxes added and usually delivered my PINs by email in a matter of minutes (but, their fraud prevention dept. do sometimes flag a transaction and require a telephone/followup call before a purchase is processed or released - good) You should be able to check it from Germany - it looked like the rates at now about 3 cents a minute for calling from USA to Germany, rounded up to 1 minute intervals, etc.

http://www.callingmart.com/products/...s/Default.aspx

Google a bit more, found some additional info on AT&T's Virtual Prepaid Calling Card (this might used to be called Worldwide Calling) - here's the link, hope it will help - http://attprepaidcard.amssupport.net/