Des,
Funnily enough, the "official" word as it came down from Gergely in the original OFM (which I tweaked for IVAP) was that it's up to you...
On IVAO, or more specifically in IVAC, the IVAO controller software, EHM is shown as "callsign EuroHarmony", hence ATC using the full VA name.
Personally, I've always used "harmony six five four" for my own callsign, since it's that little bit easier to say.
