Did you know that on average, a 15-second voice message takes about a minute to record, send, download and play? Not with Viber's Push-to-Talk feature, which basically turns your phone into a walkie-talkie, meaning that when you press the button to talk, you are recording, sending and playing the message all at once. Now, that is instant messaging.
To use Push-to-Talk, just press the microphone button on the right in any conversation, then the big button to enter Hold & Talk mode. If your conversation partner is also in Hold & Talk mode, all messages will be automatically streamed and played on either end. Otherwise, the message will be delivered as a recording as usual. You've got a 30-second time limit on your message though, so get to the point! The ring around the outside will turn red if you're waffling on too long.