Hi @OwenP54,
Greece is one of iD Mobile's Roam Free destinations, so if you're already in Greece, you can generally use your inclusive UK minutes and texts to call standard Greek landline or mobile numbers without an additional roaming charge, subject to your plan's fair-use rules.
So, calling a restaurant in Greece to make a reservation should normally be included in your allowance. Premium, non-geographic or other chargeable numbers are excluded.
One useful precaution is to set your Bill Cap to £0 before making calls if you want to prevent any out-of-plan charges.
Owethu