Home PBX: Still no CLIP on my Grandstream

I have browsed several forums on the net, read a lot of articles, but I still can't manage to get the real phone number to display on my Grandstream VoIP phone. It always shows the internal identification number from my Sipura. Maybe I should start by checking the firmware version of my Sipura. A firmware upgrade helped me to get additional features working on the Grandstream.