I have been building a push-to-talk feature called BurrowTalk into RootBadger, an independent group-based discussion service. It works over the web and in the native Android app: enter a community voice room, hold the button while speaking, and release when finished.
The idea is closer to a focused group channel than an endless public voice feed. RootBadger's threaded posts remain the durable part of the conversation, while BurrowTalk gives a group somewhere to talk live when text is too slow. Recent transmissions can also be revisited.
Android support is available now, including BurrowTalk, notifications, messages, groups, and threaded posts. An iPhone app is coming soon.
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I run the service, so this is an announcement rather than a neutral review. I would be interested in how people here use PTT over Wi-Fi or mobile data, and what matters most for making it useful rather than noisy.