WebRTC is supported in Chrome (v28+) and Firefox (v23+) on desktop. We do not support Android browsers.
The flash mode (fallback) supports Google Chrome 11+ (however, Flash is currently not compatible with Chrome incognito mode), Internet Explorer 7+, Safari 5+, and Firefox 3.6+ with Flash 10.1 or higher installed.
You can check if your browser is supported and configured correctly here: http://clientsupport.twilio.com/
If you continue to face issues with the phone channel, please email firstname.lastname@example.org.