If your current modem is ageing, the $199 new modem could be a few upgrades in one, i.e. in addition to the VOIP.
It might be (a) ADSL2+ compliant. And if you live in Perth, iiNet are massive there and offer ADSL2+ in a lot of places. So you could end up making your internet 10x faster as well as getting VOIP. That's if you don't have an ADSL2+ compatible modem already.
It might be (b) wireless. I.e. let's your computers have little antennas making connectivity throughout the house without any ugly network or USB cables. Maybe you have this already too, but that's another that thing you would tend to get in a modern modem for about the $199 mark. Mine was $199 I think on special to existing customers from iiNet and its wireless, ADSL2+ enabled, and VOIP, plus a 4 port switch in the back etc.
All depends on what ya want and what your current modem can do I guess.