Interface B does not have to necessarily know details of the PSTN implementation. When the Spirits server receives an event EDP-N, it then notifies the IP host. Redirect call, Redirect destination. Otherwise, a better way would be to make services network agnostic. Read from the charter.
Lets stick with the charter.
PINT has direct interfaces to the PSTN Gateway. It is similar with the A interface. Trigger management and data management. Event reporting only can be limiting. CS-3 is much more broader.