The PTT or Push To Talk service somehow never “caught” in Poland. It showed up a long time ago, but somehow I never saw it functional in any network here.
I always liked the idea of instantly transferring a voice message to another mobile number. I know you can do this via MMS but the HeyTell implementation shows PTT in a whole nicer way.
Because the application shows where you are (if you allow it), you can discover that a friend you are talking to is in fact in the same shopping center as you.
Available for both Android and iOS. I Highly recommend it.