It will be obviously more expensive than phoning a mobile here or a landline in Greece but I have no idea how much. A mobile call from Greece to UK is about 24p per minute so I would imagine it will be about the same for you to call Greece.
Could you not text them and ask them to text you a landline number in Greece that you could call. This would be cheaper.
EDIT: geo1986, it's 6p a minute for them to receive the call.