Here's my advice (I live in Greece)
Regarding the calling card option - check out
www.talktalk.gr. You can get this card from most kiosks, it has good rates and quality. You can use it from any payphone or fixed phone but NOT from a mobile (this is common with all cards). There are other cards which are cheaper but you get what you pay for, more or less. Avoid the Q Telecom Ya! card because calls are rounded to the next minute.
Regarding the phone/sim problem. One solution that springs to mind is to buy the Motorola C115 that Vodafone currently offers for 40 euros. For this money you get:
a) The Motorola C115 phone (very basic, dual band I believe but it has good battery life, signal and audio quality)
B) The Vodafone a-la-Carte SIM with 8 euros credit (This would usually cost 19.90 on it's own)
You should be able to get it from any Vodafone store, the a-la-Carte price list is here
http://www.vodafone.gr/live1/page.jsp?pid=...vars=0000000000. It might not be the best for your needs but as we don't know exactly what your needs are I can't suggest anything better!
Edit: The phone comes unlocked, as do all phones in Greece.