For instance, if you witness a police officer committing a crime or police brutality, would calling the police or 911 lead to an immediate response against the first officer?
Police are subject to the same law as everybody else
A report of a crime by a police officer should get the same consideration as any other report.
However, there is a perennial problem that is inherent in any organization policing its own - quis custodiet ipsos custodes. For that reason, many jurisdictions have internal or external integrity commissions or ombudsmen to deal with this problem.
Also, police, by the nature of their job, can annoy people and may therefore attract vexatious complaints.