I was a HeyTell user for quite a while, and it worked well for me. But what I really didn't like about it, was that you only got 30 seconds per message, which could fill up the inbox of the person you're chatting with. So, I decided to give Zello Walkie-Talkie a try, and I like it a lot. Not only is it easy to use, but there's no limit for how long your message can be, and it's a real-time push-to-talk app. So unlike HeyTell, you don't have to keep recording message after message. You can keep talking, and when you're done, just let go of the "Hold and talk: button. Another nice thing about Zello, is you don't have to open the app to hear your message. You can have the app closed, and when someone sends you a message, VoiceOver will announce the push notification, and you'll hear the message come through. You can not only have private conversations with your friends, but you can also create public channels for people to be part of. For example, if there's a music artist you and some of your friends enjoy chatting about, you can create a channel dedicated to that artist and invite your friends to join it.
Hope this helps. :)