Walkie Talkie Apps that actually work like walkie talkies

[Cagle]

I need to be able to send audio to another iPhone user and have them hear it right away; not via a banner notification that they have to click on before hearing my message. I've tried Voxer, Zello and other, more obscure walkie talkie apps but none seem to have this option. I know the majority of people wouldn't like to be interrupted by a voice message while listening to their iPod app, but it would be nice to have an option under settings.