If the callsign has a purely numerical tail, it is mostly the flightnumber. Knowing that number, you may use some developer APIs of airports or airlines to obtain the flight status including origin and destination. Useful APIs are provided by Amsterdam airport, Frankfurt airport, Ryanair, British Airways, Lufthansa (includes most subsidiaries/cooperations like Eurowings, Germanwings, LH Cargo, Air Dolomiti, Swiss, Brussels, Austrian, Aerologic and some connecting flights from other airlines). The amount of API requests is typically limited for developers like e.g. 500 requests per day or hour.
Another solution may be to use some database like
www.virtualradarserver.co.uk/FlightRoutes.aspx , but the data could be outdated as flight routes may change every few weeks depending on the airline.