There is another source. In some countries with rather basic ATC equipment they will issue local carriers with a block of numbers so that in theory when they are working procedural control only one local carrier will be using a similar flight number and so less potential confusion/room for error...for example in West Africa our block allocated was XXX 710-740 (others had 709 and below or 741 upwards etc.)
However this is only for ATC use and does not tie you to using those numbers on the Computer Reservation System (carriers like BA use flight numbers which in some cases bear no resemblance to the CRS Flight no.)...in fact we couldnt tie our ATC numbers to the CRS numbers as we had more actual services than ATC numbers allocated and had to duplicate their use many times on different days or different routes.