MagicJack
My brother spent a few years in Australia and set up MagicJack. They have an app as well, though I do not believe you can answer the call over cellular... You can port to MagicJack and use it that way. Or else, maybe port to Google Voice and have it forward to MagicJack... Might be a cheaper way of handling things. Plus, you can make calls back to the US for free...
fuzz