If you don't do your homework, I'll call the police!

The word POLICE is plural. It takes a plural verb: ARE, HAVE and not IS or HAS. It is usually used with THE, and it refers to an organisation, not to one person:
The police are coming! The police have a different opinion.
If you speak about individual people, you use policeman/-men, policewoman/-women or police officer(s).

How do you translate "la policía"? It depends: if you mean el cuerpo de policía, it's the police, but if you mean la mujer policía, then it's the policewoman.

HOMEWORK (and also HOUSEWORK and WORK= trabajo) are singular and uncountable. The Spanish deberes is plural, but the English word is not. It is usually used with a possessive adjective:
She does her homework in the evening.
Tiene dos trabajos = She has two jobs.

By the way, notice the title of this post: you don't call TO someone,and you don't phone TO someone. You call them or phone them: no preposition, they're transitive verbs.