The advanced userinterface of EditVoicepack does not show the available callsign, aircrafts etc. It shows the modifications that can/will be made to the default Flight Simulator voice files.
If you for example open The Netherlands, you will see "Arkefly", as EditVoicepack adds Arkefly to the voicepack. You will not see KLM as this is already included in the voice files of Flight Simulator - so EditVoicepack does not have to make any modifications for KLM to be available.
Making the list of available callsigns etc be visible somewhere in EditVoicepack is a reasonable enhancement request, but to be honest not anything I'll have time to implement in the foreseeable future. I am in the middle of a huge change to EditVoicepack as is, and unfortunately spending a lot of time on EditVoicepack is not an option right now.
You can find the list of all available callsigns in your Flight Simulator UI - unless you deselected the option to have airlines.cfg updated. You can also open airlines.cfg directly in Notepad or another text editor to see the list. If you can't find it, please let me know which Flight Simulator you use - then I can give you the exact location of the file.