A play on the usual meaning of a roll call, the Forumite roll call consists of one Forumite who calls for the roll by shouting "Forumites count off!" to which all Forumites present will shout in unison "One!"
Usually shouted out by someone organizing an activity that depends on knowing how many forumites are participating e.g. Diner Runs. Also used as a rallying cry for large numbers of Forumites as a show of solidarity. The caller asks loudly something on the order of 'we need to know how many of you are going to the Diner' and follows with "Forumites, Count Off!". The traditional answer from everybody else, refusing to obey even the most simple or logical request, is to shout in unison "ONE!".