Definitely call the police/EMS. It would stink to "save the day" only to bleed out because no one can reach a phone!
If you own it, donning some body armor would be smart.
Of course, having trained and practiced single-man room clearing would be pretty valuable!
In all, it's not a pretty situation to face. Throwing yourself into a gunfight is generally a stupid solution to the problem. Two way gunfights are nasty enough, but throw in a third choice?!
That said, sometimes it can be a "man in the arena" moment where it is more worth it to risk death/permanent disability than sit by.