Configuration Guide
CODEC PREFERENCES

Every terminating endpoint has a set of declared CODECs stored in VoIP Exchange database. Every declared CODEC is tested and committed by VoIP Exchange technical support. VoIP Exchange routing system is designed to check CODEC availability and preference prior to making a call setup. This set of tasks guarantees that connection will be established with CODECs that in both directions are equivalent or identical.
The following example is for Cisco IOS-based gateways. However, similar configuration could be applied on most H.323 endpoints.
Create a voice class to define the order of preference for selecting a CODEC when the access server/router negotiates with a destination endpoint.

voice class codec 701
codec preference 1 g723r63
codec preference 2 g729r8
codec preference 3 gsmfr
codec preference 4 g711Alaw

The optional bytes parameter sets the number of voice data bytes per packet. The number of frames per packet CODEC should not exceed 3 (three).

Apply the voice class to VoIP dial peers.
dial-peer voice 100 voip
application session
destination-pattern .T
voice-class codec 701
session target ipv4:212.53.35.34

Originating VoIP equipment could be configured with a number of audio CODECs.
It is not advisible indicating only one CODEC. Sequence could be changed to satisfy your requirements.

voice class codec 700
codec preference 1 g723r63
codec preference 2 g729br8
codec preference 3 g729r8
codec preference 4 gsmfr
codec preference 5 g711Alaw

Limitations:
Full CODEC negotiation is supported with Fast Start call setup only.
Slow Connect scenarios are limited to G.723.1 w/3fpp.
Note:
Any H.323v2 compliant endpoint should support Fast Start feature.
The default for Fast Start is enabled, which is normally the preferred configuration.
H.323 Fast Start feature usually does not require any additional configuration.