The original callsigns are not always numbers only. Already covering what has been said above, it is the airline filing the flight plan that will decide the callsign, a lot use the flight number that the passengers would see, others will have an alpha numeric callsign that is completly different to the flight number.
They normally follow a pattern if they are alpha numeric in that flights from A-B will have a set of callsigns for that route and flights from A-D will have another set.