If the person you are calling is with Optus, you are able to leave a Voicemail  / Messagebank message without calling their phone.

From a land-line, call "133-321" and follow the prompts.  You enter the mobile number and it will bring up their Voicemail / Messagebank greeting.

From a mobile you can call 0411 000 321 and it's the same process.

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23/04/2012 UPDATE:  The following will only work if you and the person you want to leave a message for are using the same phone provider.

Phone your Voicemail / Messagebank like you would normally to check your messages - e.g. 101 for Telstra and 321 for Optus. 

Listen to the prompts for the option to SEND A MESSAGE.

Enter the number, press hash and leave a message.

Your message will be delivered straight to the person's Voicemail / Messagebank without their phone ringing.