Sorry to hear your unable to call or text while in Crete.
If you can connect to a network its more thank likely roaming is active. You may need to check that international barring is switched off.
You can check roaming and barring are active when you log into Your Account .
Click on plan and device
Click the tab manage services
Make sure roaming is set to on here. If it isn't move the slider to on and click update. This will present you with a green success message. It may take up to 24hrs for this to go live on your account.
As I mentioned you can also check the call barring option to stop calls abroad isn't set to on here which I have a feeling maybe causing your issue.