This looks to me more like Neighbor A (the whistleblower) having a deep seated hatred of Neighbor B. So A decides to call the police to investigate B for something trumped up just to make trouble for B. That’s why A’s identity does need to be known, and they need to testify.
Yes, Neighbor A was probably tired of the frequent yelling and screaming going on over his head and finally decided to do something about it. All he did was tell the police there was screaming and yelling. He did NOT say what was being said and whether the screaming was pain, fear, or anger. Once on the scene, the interpretation was for the police, not the Neighbor (WB). All the WB did was alert authorities to a possible situation that needed investigating.
It’s more like Neighbor A is banging Neighbor B’s husband, and tells Neighbor B that her husband is unfaithful, without confessing her (Neighbor A’s) involvement, IMO.