Conclusion

Adding a H323 extensions directly to the Trixbox does not seem possible. Installing a gatekeeper is necessary to establish connections. Right now our H323 -> H323 calls are handled by just the gatekeeper. SIP -> H323 connections work fine. To get H323 -> SIP to work the Trixbox should probably act as gateway. The phone rings when we do this but once picked up the connection is destroyed by the Trixbox. Our guess is that we need to edit the configuration of the gatekeeper to have data streams always go via the gatekeeper, i.e. the gatekeeper serves as PROXY.