I'm really sorry, I completly misread your question above and thought you were asking about changing the dial plan, not the dialtone.
To change the dialtone,
In express talk, click "settings" and move to the "other" tab
Under ring tones, select "dialtone" and then click the "change" button
Select your own personal audio file
Press ok, and now your dialtone should change.
@[deleted], usually you would setup your Express talk phone with a VoIP service (there are also free ones), most VoIP services provide free in service calls, so basically anyone with the same service as you is free to call.
Here is a good one to try out and get started with:
http://www.faktortel.com.au
You can signup with a free account, and use express talk to register with the account and make calls through the Faktortel service. With the free account you can call anyone with another Faktortel free account. You are also able to call 1-800 numbers in Australia free of charge with the free account, even if you are not in Australia. You can also upgrade to a paid plan, and for only $7.50 a month or something you can get your own Australian phone number, 10c untimed outgoing calls to any number in Australia, very cheap rates to around the world and a lot more.