To install AT&T voices from original (v.5) DVD:

Install the Desktop first, by running setup.exe from the root of the DVD.
Install each voice by running setup.exe from the Voice1 - 4 folders on the DVD.

Then, install the VoxATC latest version from the website.

Change transition altitude to 6000 ft for UK / EU in %APPDATA%\Internal Workings\VoxATC X\VASettings.xml TransitionAltitude Value="6000".

To get version 6.xx of VoxATC to work with P3Dv3

The method is to create a series of linked folders so that it appears that an installation of P3Dv2 is installed on the computer.  When VoxATC runs it is "tricked" into thinking it is reading from a P3Dv2 installation, when it is actually reading the P3Dv3 files.  Carry out the following to set this up. 

The first step, though, is to check the registry.  I believe that VoxATC locates the P3D installation from the following key (the following is taken from an export on my PC); my installation is at D:\P3Dv3.

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Lockheed Martin]

[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Lockheed Martin\Prepar3D v2]

There should already be a corresponding key for v3.  You must create the key above for v2, but make sure it is referencing your v3 installation folder.**  One way to do this is to amend the text above within the dashes to point to your installation folder then save it as a text file with a *.reg extension.  Double-clicking that file should then import the entry into your registry.

The folders we need to create links to are

%userprofile%/Documents/Prepar3D v3 Files
%localappdata%/Lockheed Martin/Prepar3D v3
%appdata%/Lockheed Martin/Prepar3D v3
%programdata%/Lockheed Martin/Prepar3D v3

Run a command prompt with admin priveleges, and type the following commands:

cd %userprofile%\Documents

mklink /d "Prepar3D v2 Files" "Prepar3D v3 Files"

cd %localappdata%\Lockheed Martin

mklink /d "Prepar3D v2" "Prepar3D v3"

cd %appdata%\Lockheed Martin

mklink /d "Prepar3D v2" "Prepar3D v3"

cd %appdata%\Lockheed Martin

mklink /d "Prepar3D v2" "Prepar3D v3"

Do a fresh VoxATC install at this point.

Run the following (from the VoxATC folder)if the installation fails, or when necessary:

Registrar.exe (to register VoxATC with your key)
AdvancedSettings.exe (config)
JoystickSelect.exe (config)
VoxATCSettings.exe (config)
VAPanelSelect.exe (to select which aircraft to add panels to)
VoiceConfig.exe (to configure voices)
VoxATCIndexer.exe (to index scenery, aircraft, etc.  you should run this any time you make scenery changes or add aircraft)