Following up on all the discussion of "MagicJack", this is exactly what the voicemail feature of MagicJack does.
At $20 for the hardware/set-up and $20 per year for "MagicJack", you'd be spending only about $40. Then, if you just need to get your local phone provider to enable call-forwarding so that you can forward your calls to your "MagicJack" number when you want voice mail coverage.
I would be surprised if there weren't other VM-to-email services which would provide similar functionality without the need to maintain your own hardware.