Voice Conferencing

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Discord. “All-in-one voice and text chat for gamers that’s free, secure, and works on both your desktop and phone.” (https://discordapp.com)
VAIL. “Voice + email.” (http://danariely.com/resources/vail-voice-email)
Voxer. “Walkie talkie app for high-performance teams. Your team has secure, real-time communication in one powerful push-to-talk.” (https://voxer.com)