What controls the availability of Calls on the Offline Platforms if the user loses access to the Primary Account of the Call Record?
It is controlled by the value of the Call Date field. If the user does not have access to an Account, the related Calls are only visible if the Call Date is not older than 2 months.
This is hard-coded behavior and cannot be changed. This Call behavior overwrites the VMobile Object Configuration (VMOC) Settings.
CRM Help Documentation: Sync Overview