From MetroTalk –

We are sad to announce that after a request by Google, MetroTalk will be removed from the Windows Phone store on May 1, 2014. We apologize to all our users for this but the decision is beyond our control, and in fact affects all third-party Google Voice apps. The good news is that the app will remain available and fully functional for 6 more months, and you can expect bug fix updates during that time. A huge thank you to all of our users for making us one of the top downloaded and highest rated apps of the Windows Phone store. Once again, we truly apologize to everyone affected by this.

This is a decision by Google that will affect all 3rd-party apps, due to Google locking down on security in Google Voice. The Google Voice API was never publicly available. So if you use a 3rd party app for Google Voice you have 6 months notice.

Bit by bit the openness of Google as a platform is closing down and this is another step towards that end game.