This is a wrapper around the g722 codec from the spandsp library.

G.722 is an ADPCM wideband (16khz) codec. Here the NB compatibility 
mode is used - it is decoded in and encoded from 8khz.

See  
http://www.soft-switch.org/spandsp-modules.html 
 and 
http://www.soft-switch.org/spandsp-doc/g722_page.html
for details about spandsp.