Calling the police on someone means making a telephone call to the police to report someone's behavior.

Calling the police on someone means making a telephone call to the police to report someone's behavior.

I called the police on my neighbor because he was playing music too loudly at night.

Note that we say "call the police on someone" (not "about someone").

The professor called the police on two students.

The professor called the police about two students.

Be also aware that "police" is a plural noun, so we say "police are" (not "police is").

The police are trying to solve the case.

The police is trying to solve the case.

Being a plural noun, we cannot use the article "a" with it; thus, we always say "call the police" (not "call a police"). You can say, however, "a police officer".