CatchUp is an experimental mobile app for voice calls1
You can then create groups, see who's ready for a call, poke offline friends. There are some advanced features like one-on-one calls, dedicated groups, and users' current status (muted, dialing, online, etc.). Some of these features are already present in other similar apps, while there are fresh ones, that can make voice calls more fun.
It's questionable whether people need another app for voice calls when there are already plenty. Many people also report errors and crashes, so the app is far from ready at this point. If you want to give it a try, you can get it on the App Store.