Learn French: Tu as téléphoné à un copain hier ? - Did you call a friend yesterday?

It uses the passé composé, which is made of two parts. Here, we have the helping verb as (from avoir, meaning "have") and the past participle téléphoné (called). Literally, the French says "You have called."

How do I form the past participle for a verb like téléphoner?

For regular verbs ending in -er, you drop the -er from the infinitive and add an -é. So, téléphoner simply becomes téléphoné.

Why does it say téléphoné à un copain? In English we just say "called a friend."

In French, the verb téléphoner is always followed by the preposition à when you mention the person you are calling. It helps to think of it as "to telephone to a friend."

Why is there no "est-ce que" or flipped word order to make this a question?

In everyday spoken French, you can turn a regular statement into a question just by raising your voice at the end. In writing, the question mark does this job, so you don't always need formal question structures like est-ce que.