Learn Russian: Друзья тебе часто звонят? — Да. - Do friends call you often?

The verb звонить (to call on the phone) requires the Dative case for the person receiving the call. You can think of it as making a call to someone. So, the basic pronoun ты (you) changes to its Dative form, тебе.

I thought the word for friend was друг. Why is it друзья here?

Друзья is the plural form of друг (friends). It is one of the most common irregular plurals in Russian, meaning it completely changes its ending rather than following the normal rules of just adding -и or -ы.

Why doesn't the Russian sentence start with a question word like "Do"?

Russian doesn't use helper words like "do" or "does" to form questions. To ask a yes/no question, you can just use the standard statement word order and raise your pitch at the end of the sentence to show you are asking something.

Why does the verb звонят end in -ят?

The verb звонить (to call) is a Type II (or -ить) verb. When matching it with a "they" subject like друзья (friends), Type II verbs take the ending -ат or -ят. This is different from Type I verbs, which take -ут or -ют for "they".