You do not state how long you will be in Greece and Turkey, but in any event if it is at least a couple weeks you would be advised to *not* rent a cell phone. Rental rates for cell phones are exhorbitant. With what you spend on rentals you could buy your own equipment and buy local prepaid SIMs (internal phone cards) for the phone during your time in Greece and Turkey.
A used dual band handset which you would need in Greece and Turkey can likely be found on eBay for $50 or less if you're willing to settle for used equipment. Personally I'd recommend a Nokia unit such as the Nokia 3310/3410 or 3510. Prepaid in Turkey can be had for as little as an initial outlay of 13 Euros. In Greece you can get a prepaid SIM for anywhere between 9 and 19 Euros and topups for 5 Euros or so. It's likely that you'll pay around $20/day for the privilege of renting a cell phone. It's much smarter to buy a second-hand phone and either save it for future trips or sell it when you get home.
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