Explanation/Reference:
You must configure the Messaging system to use the G.711 encoding format.
Note: The G.711 format provides the highest audio quality especially when voice networks use multiple encodings and decodings. Avaya requires that you use the G.711 encoding format in Messaging systems that support TTY devices.
The G.711 encoding format uses a higher encoding rate than GSM. The G.711 encoding format therefore produces larger files and requires more storage space for messages. Messaging provides customers with adequate storage space for message playback and networking.
References: Administering Avaya Aura® Messaging, Release 6.2 Issue 2.2 (December 2013) , page 201
https://downloads.avaya.com/css/P8/documents/100172127