VOIP SIP RTP PROXY Bandwidth Optimization

i need to send call server to local voip gateway like server in USA VoIP gateway in Bangladesh server to Bangladesh Openwrt Router connected BY VPN and VoIP gateway connected to openwrt router so i send call from server its work fine but i need some one who can help me to create SIP proxy to reduce Bandwidth now per call on SIP G729 Using 30kbps but i want to make it under 10KB we need to remove or change RTP payload or you can make a SIP client to rerouting call change PTIME or if you want you can do anything else as you want we have only a openwrt router in bangladesh and you can install small package like any SIP CLIENT or any RTP remover just need to reduce Bandwidth