How to reset Samsung Officeserv SOHO IP phone?

I have a Samsung Officeserv SOHO IP telephone that now hangs on boot
forever (hangs on the screen with the five boxes). I like to try to
reset this thing to factory settings to revive it again. However, I have
found no procedure other than going into the menue (which obviously
don't work as it requires that the telephone boots). There must be a key
or key kobination to press before applying power to factory reset this
thing, but I couldn't find anything. Samsung "Service" hasn't been
helpful, either.

Anyone knows how to reset this thing?

Benjamin