A codec capability mismatch occurs when a device requiring a specific codec, such as G.711, attempts to connect across a gateway over a dial peer using limited or default codecs (G.729). This results in a busy signal.
This issue is corrected by issuing the voice class codec command, allowing multiple codec support on the gateway.
Some voice networks require support of multiple codecs to accommodate various devices. In particular, WAN connections to remote sites typically use codec G.729 to conserve bandwidth usage. Conferencing may require G.711. Devices such as Cisco Unity use G.711 by default.
On the gateway, issue the voice class codec command to allow for specified codecs. The voice class codec command is then applied to the dial peers, as shown in this example:
Router#config t Router(config)#voice class codec 99 Router(config-class)#codec preference 1 g711ulaw Router(config-class)#codec preference 2 g729br8 Router(config-class)#codec preference 3 g729r8 Router(config-class)#end Router(config)#dial-peer voice 2000 voip Router(config-dial-peer)#voice-class codec 99 Router(config-dial-peer)#^Z