How to use the 'HD' feature of the (older) Digium D series?

I note these phones are HD branded, but I’m unclear how I’d actually get them to use this feature. I tried converting my file like this:

        -y -i /input.mp3 -ar 16000 -acodec g722 -b:a 64k /output.g722

but the file is very distorted - is this even the right approach? Thanks

(I’m trying to page a sound file to desk phones)