I do not want to use a drive letter to be able to access the voicemail files, so I was thinking of embedding an FTP client into the Plugin to retrieve the files.
I'm also considering to make the plugin in a way that the use of a USB Stick as storage is mandatory (so I don't have to support multiple configurations, all those different models already give me enough "multiples"

I know a way to convert the voicemail files to wav format, so after that I can place those wav files 'somewhere', for example in the Homeseer data directory.
One thing left to figure out is how to link a voicemail wav to an incoming call.
Comments on my thoughts are always welcome, so if you think I'm making a huge mistake, please say so. Maybe the plans will change.
Any additional ideas, tips, etc. are also welcome.