I like "Lock and load. Prepare to shoot. This is not a drill." Simple, direct, and to the point. Can be spoken over the intercom and will gain immediate attention. Both from the good guys and the bad.

As far as all the other "safehaven" varients that were mentioned, a simple "Get your gun" covers all the bases. Anybody should know to secure the home when hearing that. If you are a truely prepared family, this will all be redundant anyways, because your house is always locked and you always have your gun at hand. As far as "flee safehaven" goes, the person at home at the time is in a much better position to judge the situation than a voice from afar on the telephone.