Meetme & ztdummy = echo

I am in the process of setting up a large conference server. So far the only issue I have come across is echo. I am using SIP trunks via Global Crossing. I have no voice hardware other than the server with Asterisk 1.4. I have looked at app_conference and nconference from the OpenPBX project. They seem to be lacking some of the required features like leader wait. Can anyone make a suggestion or point me in the right direction? I know there is come echo cancel options in zapatel.conf, but they seem to be for voice hardware. Normal call column for this server is 5 - 25 users in a conference.