Conclusion

After many trial and error runs and following instructions on the web we have not been able to directly add H323 extensions to the Trixbox. Installing a gatekeeper got us a bit further in the sense that a connection can be established but is destroyed as soon as the ringed extension is picked up. Right now it seems that it is impossible to directly add H323 extensions to the Trixbox. Intuitively one suspects that a software solution (using e.g. a gatekeeper) should be possible but everybody on the web using Trixbox with H323 (that is actually working) seems to have a hardware H323 solution (like Cisco Call manager).