It's all well and good for Optus to have unlimited calls to other Optus numbers as part of their Pre-paid Dollar Days deals, but how do you know which of your contacts are with Optus? With a handy "Who's who tone" that lets you know when you dial the number.
Any Optus "dollar days" pre-paid customer calling another Optus customer won't hear a traditional ringtone in their earpiece, they'll hear this. The tone lets you know if the number you've called is to an Optus number before the call is connected, so you can hang up at the last second if you think you're going to get charged.
Probably not the most exciting technological breakthrough this decade, but handy for Optus prepaid customers nonetheless.