I have a sample app that is built and it does answer calls. currently, it is simply playing some music from cstore://Sample?ns=Sample&cg=sample

I don't understand how this URI works. Why does it call cstore?

When looking at the Media that is currently loaded on the mediaserver I understand the namespace and content group but how would I call the system prompt "and-eighty-three-cents" when it is located in the mmf\system\default.

Can ED apps play system default values without calling them directly? What about TTS?

Hello - cstore mechanism is provided on AMS to keep the filed on AMS and be able to play from the location of the file under a namespaces and content group where the file is uploaded. You can use System manager to upload more files as needed.
cstore is really just content store for media files and hence the name. You can also refer to a different http location which is accessible by AMS to play files. Instead of system default, I would recommend you upload your own files and use those.
TTS is supported on ED as well.