I don't remember where I've read it, but if I'm correct, you can right-click a ringing extension to pick-up its call. (Untested feature, I never got the chance to test it out, can someone confirm it?).
But for sure, you can use the *20* command. Let's say extension 100 is ringing, well you press *20*100. And let's say you know there's an extension ringing, but you don't know wich one, you can simply press *20* to pick it up. (This is exactly what Philco refered you about).