I gave this app a try today. It's actually really good if you're after a quick, cheap solution. Also reduces the risks of someone finding your burner phone.
I made a few tests with it. It also lets someone else dial in to your temporary number. I tried dialling in a few times - it worked 4 times out of 5. Once, it just didn't ring at all. So it might not be reliable enough to rely on if you're waiting for a call from a WG.