This app was mentioned in 2 comments, with an average of 1.50 upvotes
> It would feel like push-to-talk one-way-only is an inferior way of using VOIP...
Zello supports 2-way communication. It's really flexible and has decent audio quality even over 2G. Also Zello Overlay is handy.
Might not be what you're looking for exactly, but on Android a lot of people use Overlay for Zello, maybe there's something similar to that for iOS?