I'm using the phono plugin and it works nice except that very often CORS requests seem fail and the client basically becomes unusable. In order to reproduce the issue you just need to take few calls hung up and after 2 - 3 calls received an hung ups you can see the error. I've attached a screen-shot. The issue may be reproduced directly on the kitchen demo. The issue is kinda critical because the agent basically becomes 'unavailable' due this error so the only option it has is to refresh the page from time to time otherwise it gets stuck with the error and unavailable state.
CORS often fails