Learn Turkish: Telefon etmek kolay.

Because it is acting as the subject of the sentence. Just like in English where we say 'Phoning is easy' or 'To phone is easy,' Turkish uses the dictionary/infinitive form of the verb (ending in -mak or -mek) to turn the action into a noun.

What does etmek mean in telefon etmek?

Etmek generally translates to 'to do' or 'to make,' but it is most often used as a helper verb. Turkish frequently pairs a noun with etmek to create a new action. Here, telefon (telephone) plus etmek combines to mean 'to phone' or 'to make a phone call.'

Where is the word for 'is' in this sentence?

There isn't one! In Turkish, simple statements describing what something is in the present tense often don't need a verb. For third-person subjects (like 'he', 'she', 'it', or in this case, the act of 'phoning'), you simply place the subject (telefon etmek) right next to the adjective (kolay).