G.723.1 works with Asterisk in pass-thru mode.

Other usage may require a license, depending which country you are in. Free use may be permitted for education or research.

Visit http://www.readytechnology.co.uk/open/ipp-codecs-g729-g723.1/ for an open source implementation.
Binaries can be found at http://kvin.lv/pub/Linux/Asterisk/

http://www.dspg.com/technology/LicensePricing.html

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Asterisk Codecs