You can use the profile icon (the circle with the initials at the top left) to sign out of the services or to exit the WebEx softphone application.

Sign out:
You actively sign out your current login from the WebEx services. The application restarts with the login screen and you can log in again with any login data.

Exit WebEx:
You exit the softphone application, it closes but remembers the login data and restarts with the last login data the next time you start the software.