You can drop the header bytes from 40Bytes to 2-4 Bytes! Considering that your audio is only 8kb for G.729. Must be enabled on both sides of the link.RTP runs ontop of the UDP protocol.
conf t
class-map [NAME]
match protocol rtp
policy-map [POLICY]
class [NAME]
compression header ip rtp
Interface Gi0/0
service-policy [POLICY] out