In general, when one application learns that another application wants ownership of a call, it simply relinquishes ownership of the call to that other application. Although there can be many co-owners of a call, it should be a transitory state for there to be multiple owners.
In one specific case, it is valid for an application to actively take ownership of a call owned by another application. This is when the application is instructed to do so by the user—perhaps through a user interface. For example, a fax application may be instructed by a user to break into that same user's existing voice call and use the call to send a fax. In this case, the fax application takes ownership from the previous owner, the application that was controlling the voice call.
An application can forcibly become owner of a call by taking the following steps:
Note There is no way to shield a call from another application's attempt to become an owner of it, nor is there any reason to do so. Once an application is informed that another application has become an owner, it should draw its activities on the call to an orderly close, and then relinquish ownership, because such changes in ownership are almost always done at the explicit direction of the user.