Not all installed voices can be used?

"Microsoft Helle", a Danish text-to-speech voice, is installed on my Windows PC among others.
I have the English, Danish and Spanish language packs installed on my PC (including text-to-speak and speech recognition), but only few voice options shows up with the call "GetInstalledVoices()", namely:
Microsoft David Desktop
Microsoft Hazel Desktop
Microsoft Zira Desktop
Microsoft Helena Desktop

David, Hazel and Zira are English voices, while Helena is a Spanish voice.
However "Microsoft Helle" shows up in my settings, but not when calling GetInstalledVoices().
Could you please provide an overview of which voices are available?
Why can't I choose to use "Microsoft Helle" as my text to speech voice?
Best, Peter.

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* ID: d6c6aa21-ce0f-1420-e1ed-36c4b642355c
* Version Independent ID: 55044083-e52a-299e-272a-9c4c5b97e54d
* Content: [InstalledVoice Class (System.Speech.Synthesis)](https://docs.microsoft.com/en-us/dotnet/api/system.speech.synthesis.installedvoice?view=netframework-4.7.2)
* Content Source: [xml/System.Speech.Synthesis/InstalledVoice.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Speech.Synthesis/InstalledVoice.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**