G.729a is a codec (compressor/decompressor) that is being included with Cisco Unity beginning with Unity 2.4.5. The codec is used in recording and playing back voice messages. It is an alternative to the u-Law codec, which has been used in the past. u-Law is still the default codec, but G.729a can now be selected if the administrator chooses to do so.
Sites which enable this support should not see a significant impact on CPU usage or port capacity. This will be true whether the voice board hardware supports these formats, or if the stream conversion occurs in the software.
To play voice messages recorded with G.729a codec compression using Unified Messaging, a version of ViewMail for Outlook (VMO) greater than or equal to 2.4.5.66 must be installed.
Try:
http://www.cisco.com/warp/public/788/AVVID/g729a_codec.html