Push to talk is something many app developers I think are trying to perfect. Its not easy, or else there would be more apps out there like it. Some do well, and are popular among both the Android and iOS crowd, but none have really taken off, especially cross platform push to talk.
Enter Heytell. Heytell does what others have tried and mostly failed to do, develop an app thats cross platform (Android and iOS) that allows you not only to utilize push to talk, but bring social aspects to it as well. Hit the read for a little more about this great little app.