It sounds like the default setting is for internal meetings. This is set on your WebEx admin Settings page. The URL is your WebEx site/admin. Click on the Site Settings on the left and partway down the screen check the Default MP external attendee check box. Then when a meeting is scheduled by default it will be external.
This is not normal behavior. I am not sure what is causing this problem so need to understand a little more about your environment.
I assume you have a WebEx Node for internal meetings and the internal meetings work OK? Now with the above mentioned check box checked all meetings default to the being external in the cloud though you can still schedule internal meetings.
Does your WebEx support SSO and integrated with MP for security and profile maintenance?
Are you on the network when you try to join these meetings?
Can you sign-in as the host?
If you have the WebEx Node you have internal licenses?