Currently, If the customer starts a call on the fly, it can call the first user, and his phone/Webex app rings, then answers. However, if the customer tries to bring a second user. The WebEx app of the second user does not ring and invites to join a Webex meeting instead. This behavior is by now expected and by design.
The customer would like that the WebEx app for the second user to ring without the need to set up a WebEx meeting and without using a calling service such as UCM, Webex Calling, or calling hosted by SP to achieve conference calls.