With your passport you go to any mobile telephony shop and ask for a SIM card and a mobile number certificate with your name on it.
Alternatively, make an online request to major mobile brands and you will receive your card in the mail. In this case, you provide a home address for the SIM card to be sent.
tech detail
Greece uses the same GSM system as other EU countries, Asia and Australia. But the USA and Japan use a mobile phone system that is incompatible with the European system.
Use of electronic devices
Electric power voltage in Greece is 230 V / 50 Hz and the plugs are F type. To avoid the danger of short circuit, make sure to have in your suitcase the necessary transformers / adaptors for your electronic devices or make sure that this will be provided by the relevant socket.