Learn Spanish (Latin America): Te dejan en la línea mucho tiempo. - They leave you on the line for a long time.

There isn't a specific person or group. This sentence uses the impersonal third-person plural. Spanish often uses this verb form when the agent of the action is unknown, unimportant, or a general entity like a company's customer service. It functions a lot like the English passive voice, meaning you get left on the line.

Why doesn't the sentence use por to say for a long time?

When expressing the duration of an event, English requires a preposition like for, but Spanish often omits it. You could technically say por mucho tiempo, but simply using mucho tiempo right after the verb is much more natural and common in everyday speech.

Is te a direct or indirect object here?

It is a direct object pronoun. The verb dejar (to leave) directly affects you, as you are the one being left. While te looks the same for both direct and indirect objects, knowing it is a direct object is useful if you want to swap it for a third person, like saying lo dejan to mean they leave him.

Does en la línea specifically mean on hold?

Yes, in telephone contexts it means being on the line or on hold. If you want to be even more specific about being on hold, you will also frequently hear the phrase en espera, which literally translates to in waiting.