Publish a webservice that the dialer invokes once the call has ended (if you have any influence on the dialer)
Upon calling the dialer add a PC-identifier that is returned.
Maybe you need a 'man in the middle' that dispatches these messages and sends them to the right PC. That would be a separate Mendix app. When you call the dialer call this 'Mitm' as well with the session and PC information.
If you have no influence on the dialer it will be hard.