I have just created a modified texture folder for Milviz King Air 350i for use in FSX. It is only a registration change, but I cannot change the "Radio Call" number on the VC.

The new registration number shows fine on the aircraft (external), when loaded.

I have changed the atc_id in the aircraft.cfg file and in the "Select Aircraft" details screen.

I was assuming that there would be a process in the model file or a gauge to show the various atc_id's, but that does not seem to be the case. The VC stays the same with some sort of default ATC callsign that looks like it has no connection to any of the standard textures, it just keeps showing N350K for all aircraft.

So I thought it may be located in the VC textures. I have opened ALL the texture files for both the exterior textures and the "fallback" textures where the VC is located, but cannot find anything that looks like N350K

You must be missing something. It should be via the aircraft.cfg, all the default liveries show the proper callsign on the plaque and there  isn't a specific texture for the VC in the liveries. Recheck how you have it added in the aircraft.cfg and copy what you see from other liveries.

As far as i can tell,I have the aircraft.cfg correct.........

[fltsim.0]
title=Beech King Air 350i Milviz VH-KGA
sim=350i_4
model=
panel=
sound=
texture=VH-KGA
kb_checklists=Beech_King_Air_350_check
kb_reference=Beech_King_Air_350_ref
atc_id_color=0xffffffffff
atc_id=VH-KGA
ui_manufacturer="Beechcraft"
ui_type="King Air 350i"
ui_variation="VH-KGA"
ui_typerole="Twin Engine TurboProp"
ui_createdby="Milviz"
description="The King Air in all its variants is a beautiful airplane with classic styling and graceful lines. Many of the improvements over the years have provided better aerodynamic efficiency, increased muscle under the cowlings, greater speed, upgraded avionics and electrical systems, and increased cabin luxury. In addition to duties as a corporate shuttle, the plane is also available in cargo configurations."

The VC stil shows N350K. This registration does not relate to any of the included liveries and seems to be a fixed inclusion that does not change with a different aircraft.

After a further look in the fallback texture folder I found the image including the Radio Call and it was, as I thought, a permanent call sign of N350K. Presumably a default reference to King Air 350. I changed it to suit my requirements, so all is good as I am only using the single texture. If I need to set up a new aircraft entry I will have to change it and put it in the specific texture entry for any new aircraft I use.