Back in November of 2004 while Adam Curry was looking for a way of receiving audio blog comments I messed around with exactly what Gabcast is doing. Basically using Asterisk and a call-in number to record audio for blog posts.
Just found the email threads Adam and I had regarding this but I didn't take it any further than a very simple prototype and it died due to lack of interest I suppose on both ends. Too bad we didn't take it further but nice to see that someone did.