Voxer is one of the biggest gainers in the mobile app market in the recent times.
They have mobile apps for both Google Android and Apple iOS mobile platforms. Google Android Version
The app offers push to talk capabilities. You can quickly record a short audio message that is then delivered to your friends using the service.
Since it covers both iOS and Android devices, you should be able to communicate using this with most of your smartphone owning friends.
★ LIVE Walkie Talkie, faster than HeyTell
★ Talk with friends on Android and iPhone
★ VOICE, TEXT, PHOTOS and LOCATION
★ GROUP CHATS
★ EVERYTHING is FREE
★ No annoying advertisements
★ Works over WiFi, 3G, 4G, EDGE, etc.
★ Get notifications for new messages
★ Create messages even offline
★ Play voice messages faster
★ Connect with Facebook friends on Voxer