Setup Asterisk (Freepbx) as voip provider for 3cx

I see some people discussing this on the forums but am not sure how to make it happen. I need all of my 3cx boxes to connect to my asterisk (freepbx) server. I’ve got a voip provider setup on the asterisk server and am able to dial out and in just fine. I’ve heard it has to be done by IAX 2 but couldn’t get it to work. 3cx gives me a (no auth) forbidden 403 when trying to connect to asterisk box. Asterisk sees the 3cx box because it’s reachable and 3cx sees asterisk box saying registered. ???