Originally posted by GUB What does no call sign mean?
It's interesting that a commercial machine like that wouldn't be using one. The callsign is the flight identity for flight plan filing and radio comms, and has written and spoken versions for those mediums.
Airline operators will use a version of their scheduled flight number like "New Zealand Four Five One" aka NZ451 will be an AKL-WLG fight. "New Zealand Eight Two Five Lima" aka NZ825L would be one of the Dash-8s, ex Air Nelson, who used to use "Link Eight Eight Two Five" for such a flight before Air NZ moved them onto their own operator certificate.
Private aircraft will mostly just use their registration in phonetic form, so ZK-ARJ would use "Alpha Romeo Juliet". Some have
special callsigns that are registered for the aircraft, particularly warbirds. For example the Nanchang based at Wellington goes by "Nanchang 55" (in reference to its fake tail number). These are kind of like personalised plates, though the actual registration is still in the form ZK-MAO for that Nanchang and the aircraft is marked as such.
Other commercial stuff would generally use some variation of the company name. I found a list of
registered names in NZ but I guess PHI is a multi-national so is probably defined elsewhere.
The other option here is someone forgot to set the callsign into the aircraft transponder so FR24 have nothing to go on. FR24 display three pieces of information when they have them. Registration (ZK-NET), Callsign (NZ825L), and flight number (NZ8825).