Going overseas with a mobile phone?

In theory, a GSM/3G phone should work everywhere in the world. In practice, there are a few hindrances - mainly that different regions of the world use different frequency bands, and not all phone sold everywhere will be able to work on all of those frequency bands. But from what I could Google in a few minutes, you should be ok with the S7 (at least the versions that are sold in Europe, EAN 8806088271088, 8806088321554, 8806088192918, 8806088272061, 8806088388786).

Which leaves the problem of the SIM. Either way will work - getting a SIM in Australia and usng it in Greece, or getting a SIM in Greece and using it to call Autralia. And either way it will be expensive, just differently. Using the Oz SIM in Greece will hit you with high roaming charges - check these with your provider in Oz. Using the Greece SIM to call Australia will hit you with huge long distance charges. Cheapest way would be to set up a messenger (that uses data instead of voice) and get a SIM with a reasonable data tariff in Greece (DON'T use a data tariff with a foreign country SIM, these will bleed you dry!).

You could do that through roaming which is fairly expensive or I’d personally stick with WiFi connections to handle calls/texts.

The phone would need to be unlocked in order for it to accept a local Greek SIM card assuming you are planning on buying the phone in Australia.