rusuden wo kiitara, sensei kara 「ato de orikaesite kudasai」 to haitte imasita.

Yes. 留守電（るすでん） is a casual shortened form of 留守番電話（るすばんでんわ）, which means answering machine or voicemail.

In everyday Japanese, people very often say 留守電 instead of the full 留守番電話.

So here, 留守電を聞いたら means something like when I listened to the voicemail.

Because 聞く means to listen to or to hear, and the thing being listened to is marked with を.

So:

• 留守電を聞く = to listen to the voicemail
• 音楽を聞く = to listen to music
• 話を聞く = to listen to a story / what someone says

In this sentence, 留守電 is the direct object of 聞いたら.

Grammatically, ～たら can mean if, but very often it means when or after in a past-event sequence.

Here, 留守電を聞いたら does not mean a hypothetical if I listen to the voicemail. It means:

• when I listened to the voicemail
• after listening to the voicemail

So the sentence is describing what the speaker found out upon listening to the message.

から means from, so 先生から means from the teacher.

The sentence is saying that the voicemail message was from the teacher. That is why から is the natural choice.

Compare:

• 先生からメッセージが入っていた = There was a message from the teacher.
• 先生が would make teacher the grammatical subject.
• 先生に would usually mean to the teacher, which is not what this sentence needs.

後で（あとで） means later.

折り返してください comes from 折り返す, which literally means something like to turn back, but in phone-related Japanese it means to call back or to return a call.

So 後で折り返してください means:

• Please call me back later.
• Please return the call later.

It is a common expression in phone messages and business Japanese.

The particle と marks quoted content.

So the structure is:

• 「後で折り返してください」と入っていました
• literally: It said, “Please call back later.”

This と works like quotation marking after direct speech or reported content.

You can think of it as connecting the content of the message to the verb that follows.

Yes, the basic meaning of 入る is to enter or to go in, but in Japanese it is also used for things like:

• a phone call coming in
• a message being received
• a reservation being made
• information being included

With voicemail, メッセージが入っている means there is a message or a message has been left.

So here, 入っていました means something like:

• there was a message
• a message had been left
• it said ...

This is a very natural expression in Japanese.

The subject is omitted, which is very common in Japanese.

Something like メッセージが or 留守電が is understood.

So the full idea is roughly:

• 先生から「後で折り返してください」というメッセージが入っていました。

But Japanese often leaves that unstated because it is obvious from context.

This is a very common and important point.

～ている does not only mean an action in progress. It can also describe a resulting state.

So:

• メッセージが入っている = a message is there / has been left
• メッセージが入っていた = there was a message / a message had been left

In this sentence, the speaker listened to the voicemail and discovered that the message was already there. That is why 入っていました sounds natural.

It gives the sense of a state that existed when the speaker checked.

In 後で, the で is part of a set expression meaning later or afterward.

So:

• 後で電話します = I’ll call later.
• 後でやります = I’ll do it later.

You do not need to translate the で separately here. Just learn 後で as a chunk meaning later.

Yes. ～てください is a polite request form, so 折り返してください is polite.

It is very normal in voicemail messages, workplace Japanese, and polite everyday speech.

That said, it is still a fairly direct request. Depending on the situation, Japanese can become even softer, for example:

• 折り返していただけますか = Could you call me back?
• 折り返しお電話ください = Please return the call.
  (more formal/business-like)

So the version in the sentence is polite and natural, especially for a message.

Sure:

• 留守電を聞いたら、 = when / after I listened to the voicemail,
• 先生から = from the teacher
• 「後で折り返してください」 = Please call back later
• と = quoting particle
• 入っていました。 = there was a message / it said / a message had been left

So the flow is:

When I listened to the voicemail, there was a message from the teacher saying, “Please call back later.”

It is overall neutral-to-polite.

Points that make it polite/natural:

• 折り返してください is polite
• 入っていました uses ました form

But it is not highly formal or stiff. It sounds like normal spoken or written Japanese when explaining what was on a voicemail.

A more casual version might be closer to:

• 留守電聞いたら、先生から「後で折り返して」って入ってた。

A more formal version could be:

• 留守番電話を確認したところ、先生から「後ほど折り返しください」とメッセージが入っておりました。