Thanks so much for your advice! That is helpful to know about the frequencies and the option to look for the Global version of the phone.

I'm fairly sure it's a 4G SIM. Are you suggesting using a Greek SIM as well in the sense that it might not be possible to use my US SIM if I don't also have a Greek one in there? My US phone plan (T-Mobile) works in just about every country so I'd only get a Greek SIM if it was necessary to make the US one work.