For companies that sell their services solely based on VoIP in the middle, this move by Optus isn't good news.
In essence what Optus is doing and saying to their customers is if you're their contractee, all calls go over the Optus network and the Optus long distance network. No dialing around. No call backs, no VoIP in the middle. No cheap calling.
Well with three other carriers in Australia not behaving like Optus the answer is simple. Do the math. Figure out the break even point. Terminate the contract at that point, then switch and keep using the discount calling services with another carrier.