Thank you al3x1k0 and also dulciusexasperis for the excellent information, I have researched that my USA carrier (ATT) uses the Vodaphone network in Greece, so for calls it seems I can buy that plan from ATT.
It's the data plan that is my main concern as unfortunately I have to also work during my 2 week vacation on the Aegean islands. I have found an offering for an Unlimited 4G Pocket MiFi Internet service at Euros 10/day.