You can’t call trigger explicitly!
A trigger is a named PL/SQL unit like stored procedure in a database which automatically invokes whenever a special event occurs at the database site For example; a trigger can be invoked automatically when a row is inserted into a specified table or when certain table columns are being updated.
Unlike a stored procedure, you can enable and disable a trigger, but you cannot explicitly invoke it. While a trigger is enabled, the database automatically invokes it—that is, the trigger fires—whenever its triggering event occurs. While a trigger is disabled, it does not fire.
Trigger implicitly fires when you perform any manipulation on the table (like Insert, Update, and Delete).