Para realizar a captura direto na interface do Voice Gateway, segue abaixo o procedimento.
!Configure a captura
VOICE_GATEWAY#conf t
VOICE_GATEWAY(config)#ip traffic-export profile pcap mode capture
VOICE_GATEWAY(conf-rite)#bidirectional
VOICE_GATEWAY(conf-rite)#exit
!Defina a captura na interface desejada
VOICE_GATEWAY(config)#int GigabitEthernet0/1
VOICE_GATEWAY(config-if)#ip traffic-export apply pcap size 10000000
VOICE_GATEWAY(config-if)#exit
VOICE_GATEWAY(config)#exit
!Para iniciar a captura
VOICE_GATEWAY#traffic-export inter gig 0/1 start
!Para encerrar a captura
VOICE_GATEWAY#traffic-export inter gig 0/1 stop
!Para exportar a captura para um servidor externo
VOICE_GATEWAY#traffic-export inter gig 0/1 copy tftp
!Para limpar a captura do buffer
VOICE_GATEWAY#traffic-export inter gig 0/1 clear
Mais informações: http://www.cisco.com/c/en/us/td/docs/ios/12_4t/12_4t11/ht_rawip.html