What obligation does a police force have to respond to a call about a man armed with a gun?

The police were obligated to respond to the call and investigate the complaint of a man armed with a gun At night in a high crime area they found the defendant who matched the description given by the caller. When, upon inquiry by the police, the defendant gave an unintelligible, unresponsive reply, the officer could then make a limited pat down search in the nature of a frisk, not to discover evidence of a crime, but in order to pursue his investigation without fear of violence (see, People v. Stewart, 41 N.Y.2d 65, 390 N.Y.S.2d 870, 359 N.E.2d 379).