TNG call capacity is memory based. For more details regarding call capacity please refer to forum post:
How many calls can my TNG handle?Once 99% capacity is reached it will auto-purge down to 80% capacity in use. This is an automated process.
You can only use the reports feature in TNG to access calls in your 'live' TNG. Once calls are purged, they are no longer available for the TNG Reports feature.
If you did setup TNG archiving, only the TNG reports configured to be archived will be available. For more details regarding archiving and scheduled reports refer to forum post:
TNG and scheduled reports/archiving reportsTo access the archived reports, use the icon available on the TNG Home page. It will be listed with the other options(for example General Settings, Synchronization) in the centre of the screen. If you mouse over the icon it will display a message for archiving and/or scheduled reports. Click on it to open the archived reports and search for the required report.