A trick we sometimes did with those phones (although I wouldn't recommend it) was to program the dss add-on buttons, although the dss was not actually connected the buttons were still programmable and we would use them for the call keys, this meant the customer effectively got more buttons, the down side was if you didn't have trunk keys and put a call on hold, it was placed on a call key which did not have a phyiscal button.

Samsung, Ericsson, NEC, Nortel Certified Installer