Add echo to Asterisk application(repost)

We have a small Asterisk application which modifies the voice of the caller while making a call. Currently, the caller only hears the voice of the called party, and the called party hears the modified voice of the caller. We want to modify the code so that the caller will hear both the voice of the called party and his own modified voice. Please quote and I'll provide the Asterisk source code to be modified. Our Asterisk version is 1.4.21 on a Debian. Thanks,