There was a problem with the 0.5 release which resulted when two developers were working on it simultaneously (z2n and Brontide). Brontide is making significant improvements to the application, and is getting near to releasing the updated version. See his recent posts in this tread for details.

In the meantime, you are best off reinstalling a prior version (0.4 seems to work for some people, but I think requires that you close and reopen it each time you want to make a new call, due to the concatenation/busy signal issue), or waiting for a little while until Brontide and z2n can complete the new version.