If only for a short trip, shut off all data, don't make phone calls. Notify Telstra that you need International Roaming.

Text is still cheap, and receiving phones calls isn't too expensive...

Take a netbook/smartphone and a cheap light headset/mike so you can hear. Use free wifi coffee shops / maccas and use skype . $20 credit buys heaps of time. This saved us a few times in europe when we were finding wwoofing farms and calling heaps of people, where cheapest sim was $50 for 20 mins airtime or 20x 1 minute calls to mobiles!! We were sitting outside maccas in france in the street making calls, with the speaker up to your ear trying to hear... that's why you need the headset. Saved us hundreds though.
As aqua says UK is great, buy a 5 quid sim get 5 hours international or something stupid. Pity the rest of europe isn't like that.

Skype - 3 cents a minute to land lines and free between fellow skype users. Bargain. A bit more expensive when calling mobiles though - 25 cents a minute I think.

I use skype on my phone whenever I am away and call my family in NZ all the time with it -it rocks.

If you do get a smart phone you can also use things like facetime and viber as well which give u free call options as well. I find Viber a little unclear sometimes so mainly use skype instead.