Andy,
I've only just seen this thread so sorry for the late reply. If you haven't solved this already you might want to go to
the EditVoicepack website where you can download an FSX compatible version of this essential software which allows you to add ATC callsigns (and much more) to the sim. I don't know whether anyone has already added our callsign to the FSX database there but, if not, I'm sure we can transfer the files that we use for FS9 to the new version (as they are basically WAV sound files).
Let us know if this helps