I was checking out the FAQs for your point around changing device - they have in the FAQ page - (Sorry I'm new so not allowed to post a link :( ) this info:
I have changed my device, can I reinstall the app? What happens to my voicemails?
If you change devices, follow the steps below:
- The app will NOT download your current custom greeting and old voicemails to your new device, you'll need to record a new custom greeting and save important voicemails from the ‘share’ feature.
- Delete Spark Voicemail from your old device.
- Install and activate the app on your new device.
So at least they did indicate what happens, but would be a good feature for them to support in the future maybe, possibly a privacy concern on device change perhaps?