Hello, IMO it will probably be a bit cheaper and less time to buy one for about 10 or 15 bucks, But if you're doing it as a project or hobby, I would suggest getting your hands on a Voice Recordable Chip from Radio Shack or other Electronic Outlet, and then put together a simple schematic that will allow you to tie it into your phone system...
I believe many of these type of chips allow for multiple recording points which will serve good for numerous messages.