Picture by Karl Barron
Optus has introduced a special call tone/message for customers using its Dollar Days prepaid plan. If they ring a customer on another network, they hear the tone (you can listen to it here as an MP3). It's an interesting solution, but not an ideal one: if you were only planning to make the call if it was free and you don't hear that message, you'd have to hang up in a rather rude hurry.
Vodafone and 3 offer the Who2Call application to identify other users on those networks, which is handy, though the information is only updated weekly. Another option would be to keep a note on your phone contacts of who uses 'your' network. But in practice, I suspect lots of people take advantage of the free calls with a handful of friends and family, and otherwise treat it as a useful and unexpected bonus.
Which approach do you use to make sure you get the best value from free calling on the same network? Tell us in the comments.