Learn Finnish: Soita poliisille heti! - Call the police immediately!

Soittaa (to call) is a Type 1 verb. To make the command form for one person, you find the 'I' form of the verb (minä soitan) and remove the -n. Notice that the strong double -tt- in the dictionary form weakens to a single -t-, giving us soita.

Why does the word for police have the -lle ending here (poliisille)?

The ending -lle means 'to' or 'onto'. In English, we 'call the police', but in Finnish, you call to someone. With the verb soittaa, you always add -lle to the person or place you are calling on the phone.

Do I need to include the word sinä (you) before the command?

No, you usually leave out sinä when giving commands in Finnish. The command form soita already implies that you are telling one person to do it. Adding sinä would sound unnatural here.

Is soittaa used for any kind of calling?

No, soittaa specifically means calling on a telephone, ringing a bell, or playing a musical instrument. If you mean calling out with your physical voice (like shouting for help), you would use a different verb, such as huutaa.