Asterisk act as client with Cisco

Hi all!
When my Asterisk act as client with a Cisco-SIPGateway/IOS-12.x gateway, I have the problem that I hear not so well Music on Hold because VAD and CNG are enabled on the Cisco.
There is a method for say to Cisco that I don’t want to use VAD and CNG???

Thank you, and sorry for my poor English!

dial-peer voice 123456 voip
destination-pattern 123456
session protocol sipv2
session target ipv4:
dtmf-relay cisco-rtp rtp-nte
codec g711alaw
fax protocol pass-through g711alaw
no vad