You will find the details for vodafone here
http://www.vodafone.gr/live1/page.jsp?pid=0101020300&vars=0000000000
Sim card seems to be about 17.5 euro, if I remember rightly this has a few euros credit included ( 5 or so I think)
Costs to the UK look like about 60p min
You will probably find that if you buy payphone cards when in Greece these usually seem to work out cheaper to use.