If you wish to use your mobile/cell phone while in Greece and you are not from an EU country, you’ll need to either pick up a local SIM card or activate global roaming before you leave home.
If you do choose to use global roaming, ensure you check what charges are associated with international data and calls as these are often prohibitively expensive.
The three most popular Greek service providers are Wind, Vodafone and Cosmote, with Vodafone and Cosmote having the best coverage in more remote areas, particularly the islands. If you’re heading north towards Thessaloniki, Cosmote will offer the best coverage.
Please note that there are some areas of Greece where coverage will be sporadic or non-existent, particularly outside of the cities and on some of the more remote islands.