Are you using calibre to basically copy a file from your computer to your Kindle. If so, then no it cannot be Whispersynced with another device. If you are using calibre to send the file to the Amazon Cloud, assuming calibre can do that, then yes the book can be downloaded and Whispersynced to multiple devices including your iPhone. The key here is that the book must be sent to the Amazon Cloud, not directly to the Kindle.