If you primarily use your phone as a data device, why not move your calls to VoIP and save on call costs?
Optus CEO Paul O'Sullivan recently hailed the late Steve Jobs as being responsible for "moving the mobile from the human ear to the eye". Indeed, most APC readers would spend far more time looking at their phone screen than holding it up to their ear.
If you fit into that category, why pay for phone calls in your plan that you don't really need? By getting a data-only plan, you can use VoIP for calls on your mobile, which will use up a very small amount of your data and will cost you next to nothing.
There are many good apps for VoIP -- some APC has had good success with include Acrobits Softphone ($3.99) on iPhone and Sipdroid (free) on Android. If you have a phone running Android 2.3 (Gingerbread) or newer, you can also use the built-in VoIP system.
Of course, the downside of using a data SIM in your smartphone exclusively is that depending how tenacious the app is at registering with the SIP server, you might not always receive calls reliably.
On the iPhone, with VoIP apps supporting Apple Push technology, the cloud server can maintain the registration with your provider's VoIP server, regardless of whether the app is running on your phone. When someone rings, a push alert appears on your iPhone and tapping it will launch the VoIP app to take the call.
With a data-only SIM, you generally will still be able to send and receive SMSs. One tip for making VoIP work well over mobile data connections is to choose the Internet Low Bit Rate Codec (iLBC), which is designed to use very little bandwidth and handle jitter (variation in the amount of delay on the connection) and packet loss better than other codecs.
However you'll need a VoIP app and VoIP provider that both support the use of iLBC. If you don't have both, you can try using G.729 -- a commonly supported low bit rate codec, though it's not as good at handling packet loss and jitter as iLBC.