When creating your account you have the option to take a picture that will become your in-call avatar and be used to identify you on a call. Currently, on iOS devices, if you have not yet taken a photo, you will get stuck on the screen pictured below (smile for the camera).
To resolve this state, simply rotate your device to landscape mode. You will then be able to snap a photo and join calls like normal.
This issue is being worked on and will be resolved soon in an upcoming release.