haha kara rusuden ga haitte imasita.

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）.

In everyday Japanese, 留守電 can mean:

• an answering machine / voicemail system
• a voicemail message

In this sentence, it means a voicemail message.

So 母から留守電が入っていました means something like:

• There was a voicemail from my mother.
• My mother had left a voicemail.

When Japanese speakers talk about their own mother to someone else, they often say 母（はは）.

• 母 = my mother / one’s own mother, in a more neutral or humble way
• お母さん = mom / mother, often used:
  • when addressing your own mother directly
  • when talking about someone else’s mother
  • sometimes when speaking casually about your own mother inside the family

So in this sentence, 母から is the natural way to say from my mother.

Here, から means from and marks the source.

So:

• 母から = from my mother

It is not the because/since meaning of から here.

In this sentence, it tells you where the voicemail came from.

Because the verb here is 入る（はいる）, which is an intransitive verb.

• 入る = to enter / to come in
• 入れる = to put in / to insert

Japanese treats this expression as if the voicemail came in or was received, so the voicemail is marked with が, not を.

So:

• 留守電が入る = a voicemail comes in / is received

This is a very common Japanese pattern, even though English would usually say something more like I got a voicemail or someone left a voicemail.

入っていました is the polite past form of 入っている.

Breakdown:

• 入る → to enter / come in
• 入っている → has come in / is in / is there
• 入っていました → had come in / was there

In this sentence, it does not sound like the speaker watched the voicemail arrive at that exact moment. It sounds more like:

• the speaker checked their phone/answering machine
• and found that a voicemail was already there

So the nuance is often:

• There was a voicemail from my mother
• I found that my mother had left a voicemail

This is a very common learner question.

入りました

This focuses more on the event coming in.

• 留守電が入りました = a voicemail came in

入っていました

This often describes the resulting state that the speaker noticed later.

• 留守電が入っていました = there was a voicemail / a voicemail had come in

So 入っていました often feels more natural when the speaker is reporting what they discovered after checking their phone.

It has a nuance like:

• When I checked, there was already a voicemail from my mother.

Yes, very possibly.

Japanese often leaves out information that is obvious from context. In this sentence, what is omitted could be something like:

• on my phone
• on the answering machine
• for me

For example, the full idea in English might be:

• There was a voicemail from my mother on my phone.
• My mother had left me a voicemail.

Japanese does not need to state all of that if the situation is already clear.

Yes, it is a very common expression.

Even though 入る literally means to enter, Japanese often uses it for things like:

• calls coming in
• messages arriving
• voicemail being left

So 留守電が入る is a natural set phrase meaning:

• to receive a voicemail
• for a voicemail to be there

It may feel a little indirect compared with English, but it is very normal Japanese.

Yes, it is polite because of いました.

• plain style: 母から留守電が入っていた。
• polite style: 母から留守電が入っていました。

The vocabulary itself is neutral, and the -ました style makes it appropriate for normal polite conversation.

The structure is:

• 母から = from my mother
• 留守電が = a voicemail
• 入っていました = had come in / was there

So the literal structure is roughly:

• From my mother, a voicemail had come in.

More natural English:

• There was a voicemail from my mother.
• My mother had left a voicemail.

Usually, no.

留守電 specifically refers to voicemail / an answering-machine message, not just a missed call.

If it were only a missed call, Japanese would usually express that differently, such as with wording related to:

• a missed call
• a call that came in while you were away
• call history

So this sentence strongly suggests that the mother actually left a voice message.