Radio Altitude Height Callouts

This logic behind the sounding of Rad Alt height callouts was a topic brought up and answered on the PPRuNe site, but I am still none the wiser...My old 747-400 notes mention that the RA height callouts are armed when the aircraft climbs above 1000' and each callout will only sound once below this height (not good if you're a bad flyer or flying over undulating ground). My notes also mention that the landing gear lever should be in the down position. However, there is no mention of this landing gear lever input in either the 767 or the 747-400 Boeing Maintenance Manuals. Also, considering the fact that there can be callouts programmed to sound above 1000', how do these fit into the 1000'/one time only/gear down logic? For example, there can be a 2500' callout. The landing gear may not be extended until below this height. Hopefully the experts here can tell me what really goes on in those mysterious (RA) black boxes? :-)Thanks. Cheers. Ian.