I can only imagine what it feels like to lose the only voice recording you had of him and I think this is a tough situation for Sprint and probably more on your end to get what you want accomplished done (restoration of the line and voicemail).
I don't know if they keep back-up copies of voicemail recordings, but if they do, it's just a matter of getting to the right person who understands what you need. If they don't, as hard as it may be, you may have to understand there is never going to be a solution. Accidents happen, as you know all too well, no one meant for you to lose the line, it just happened.
I don't think this is lawsuit worthy as another poster mentioned, but a well tailored letter to the CEO, Chief Marketing Officer, etc. may be well deserved to explain what it meant to you to have his voice available and what it felt like to lose that forever.
A letter to ceo and It's my new mission to spread the word how AWEFUL sprint is and cold hearted. Yes I understand accidents happen you dont need to remind me I LIVE with that thought daily thanks for the lovely reminder so thoughtful. Also You have no more proof that "no one meant for you to lose the line, it just happened" than I have proof they meant for this to happen!!!!!!!!!