So, just to let you know, I have successfully integrated a Cisco 9951 into the 3CX System !!
Considering that 3CX Server is more or less providing the same SIP connection base than a FritzBox, I was able to modify the settings xml file from this thread to a working configuration for 3CX: http://www.ip-phone-forum.de/showthread.php?t=258141, copied the configuration file to the root folder of a pumpkin TFTP and rolled it out to the phone... Once you know what is going on, it´s rather easy and straight forward.