Sorry Carter, quite some time ago, I changed the format of the header block in the recorded voicemail. I think you need to strip off the first 57 bytes file to generate your .dat file, not the first 56. The bottom line is the resulting .dat file should have a file size of an exact multiple of 9. There are 9 bytes in a 20-millisecond block of ambe data, so if you don't cut the file in exactly the right place, it's useless.
Try that and get back to me and if it works, I'll update the README and push it up to the repo.
Tom Early