SIP Early Media / Delayed Meida (Asterisk <--> Cisco)

Hi,

in my specific setup, i have cisco 2851 as my pstn gateway and from there sip call going to asterisk. everything is working fine, but noticed asterisk server is using early media sip communication (rtp stream before session established). any idea how to change this setting on asterisk to convert to delayed media sip communicaiton?

thanks
ldp