The Eic_CallIdKey attribute holds that value for a call (there is no equivalent for a ConferenceId)
So, in the CustomCallDisconnect handler you can access the Eic_CallId, Eic_CallIdKey, and Eic_ConferenceId attributes.
Another attribute you would probably find useful is the CallLog attribute. It contains an entry that shows when the call is added to a conference, including the ConferenceId. Parse that attribute to find each occurrence of "conference" and pull out the 10 characters following the space after conference.
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