By default in Cisco Unity 3.1 and 4.0, Cisco Unity TAPI Service Provider (TSP) ports that register with Cisco CallManager have capabilities for both G.711 and G.729. You can disable on-server transcoding so that an external transcoder can be used by Cisco Unity.
Ensure that the latest version of the advanced settings tools has been installed on the Cisco Unity server. This is available for download from Cisco Unity Tools-Advanced Settings Tool. Perform one of these procedures:
On the Cisco Unity desktop, double-click the Cisco Unity Tools Depot icon. From the Cisco Unity Tools Depot, use the advanced settings tool to set Audio - Enable G729 codec support to 0.
Manually add the HKEY_LOCAL_MACHINE\SOFTWARE\ActiveVoice\AvSkinny\ DWORD:G729 Enabled key to the registry. You need to set the G729 Enabled value to 0 to disable, and 1 to enable.