You need to create a mix for the "Send In" from the mixer that contains no caller audio (mix-minus). Otherwise you'll get phasing of the caller audio or feedback. The caller's signal will come into your mixer on one of the channels from the "Caller Out" jack of the DH20. You need to make sure that signal doesn't get feed back to the caller via the Send In jack. Best way is to use an aux send on your mixer to feed it. Anything you want the caller to hear from the mixer (host mic, music inputs, etc), you dial up on the aux send feeding the Send In. That way you'll avoid feedback loops so long as you NEVER turn up the aux send feeding the DH20 on the channel being fed by the Caller Out jack.
