Learn German: Rufst du den Arzt an? - Are you calling the doctor?

The dictionary form of this verb is anrufen (to call). It is a separable verb. In a question or a standard statement, the main part of the verb (rufst) takes the normal verb spot, and the prefix (an) gets kicked all the way to the very end of the sentence.

If I wanted to say the statement "You are calling the doctor." instead of asking a question, where would the an go?

It still goes at the very end! In a statement, the subject comes first and the verb is in the second position, so you would say: Du rufst den Arzt an. The separable prefix always waits at the end of the clause.

Does anrufen mean something different from just rufen?

Yes! The basic verb rufen means to physically shout or call out to someone. When you want to say "to call on the phone," you must use the separable verb anrufen.

Why is it den Arzt and not der Arzt?

The word Arzt (doctor) is a masculine noun. In this sentence, the doctor is the direct object (the person receiving the action of being called). For masculine nouns in the direct object position, which is called the accusative case, the word for "the" changes from der to den.