> The key detail for me is that if you delete the call from the log on any device, the next sync will delete it in iCloud.
I highly doubt that it is a hard deletion of data. My guess is that it would be a soft delete, so your call log won't show up on your iPhone, but the data will be retained on Apple's servers.
> One way call logs will disappear from the cloud is if a user deletes a particular call record from the log on their device; then it will also get deleted from their iCloud account during the next automatic synchronization.
I highly doubt that it is a hard deletion of data. My guess is that it would be a soft delete, so your call log won't show up on your iPhone, but the data will be retained on Apple's servers.