Mumble is an open source, low-latency, high quality voice chat software primarily intended for use while gaming, but that you can use for any audioconference / chatting.
No user account is needed to use it.

Several Clients

You can use Mumble from your desktop or from your phone.

Desktop: Mumble

• Access channels from you desktop.

Android: Plumble

• Access channels from you Android device.

iOS: Mumble

• Access channels from you iOS device.