denpa ga yowai toki ha, tatemono no soto ni deru to rusuden mo kikiyasui desu.

Here, 電波 literally means radio waves, but in everyday Japanese it often means signal or reception, especially for phones.

So 電波が弱い means something like:

• the signal is weak
• the reception is poor

It does not usually mean a science-style discussion of electromagnetic waves here.

In 電波が弱い, が marks 電波 as the thing that has the property weak.

So the structure is:

• 電波 = signal
• が = subject marker
• 弱い = weak

This is very natural in a descriptive clause like 電波が弱いとき = when the signal is weak.

If you used は, it would sound more like a separate statement about the signal as a topic, rather than a smooth description inside a larger phrase.

This part breaks down like this:

• 電波が弱い = the signal is weak
• とき = time / when
• は = topic marker

So 電波が弱いとき literally means the time when the signal is weak, or more naturally, when the signal is weak.

Then は makes that whole time phrase the topic:

• As for when the signal is weak, ...

A useful thing to notice is that 弱い directly modifies とき. Japanese often puts a whole clause before a noun this way.

建物の外 means outside the building.

Breakdown:

• 建物 = building
• の = links the nouns
• 外 = outside

So 建物の外 is literally the building's outside, which in natural English becomes outside the building.

In this sentence, に marks the destination:

• 建物の外に出る = go out to outside the building

Japanese often uses に with movement verbs to show where someone goes.

You may also see:

• 建物を出る = leave the building

The difference is:

• 建物を出る focuses on leaving the building
• 建物の外に出る focuses on ending up outside the building

Both are natural, but this sentence emphasizes the place you move to.

No. Here, と is not and. It is a conditional/result marker.

So:

• 建物の外に出ると = if/when you go outside the building
• or when you go outside the building, ... happens

This と is often used for things that are regular results, natural consequences, or things that generally happen.

So the sentence is giving a kind of general rule: when the signal is weak, going outside leads to better hearing of the voicemail.

They do both relate to when, but they do different jobs here.

• 電波が弱いときは sets the overall situation: when the signal is weak
• 建物の外に出ると gives the action and result: if/when you go outside, then ...

So the sentence has two layers:

1. In the situation where the signal is weak,
2. if you go outside the building, voicemail is easier to hear.

This is completely normal in Japanese.

留守電 is a casual short form of 留守番電話.

Depending on context, it can mean:

• an answering machine
• voicemail
• a voicemail message

In this sentence, it most likely means voicemail or voicemail messages.

も means also, too, or sometimes even.

So 留守電も聞きやすいです suggests something like:

• voicemail is also easier to hear
• you can hear even voicemail more easily

This often implies some unstated comparison, such as: not only calls, but voicemail too.

If the sentence had 留守電が聞きやすいです, it would sound more neutral and simply state that voicemail is easy to hear.

聞きやすい means easy to hear or easy to listen to.

It is formed from:

• 聞く = to hear / to listen
• verb stem 聞き
• やすい = easy to do

So:

• 聞く → 聞きやすい
• literally: easy to hear/listen to

This pattern is very common:

• 読みやすい = easy to read
• 食べやすい = easy to eat
• 分かりやすい = easy to understand

The opposite is にくい:

• 聞きにくい = hard to hear

Yes. That is normal.

聞きやすい is an い-adjective, and in polite speech you can add です:

• 聞きやすいです = polite
• 聞きやすい = plain/casual

What you do not do is add だ after an い-adjective in this kind of sentence.

So:

• 聞きやすいです = correct
• 聞きやすい = correct
• 聞きやすいだ = incorrect

This is a good nuance question.

• 聞く is more like listen to / hear intentionally
• 聞こえる is more like be audible / can be heard naturally

Because voicemail is something you actively listen to, 聞きやすい makes sense.

That said, depending on context, a native speaker might also choose something like:

• 聞こえやすい = easier to hear
• 聞き取りやすい = easier to make out clearly

So the sentence is understandable as written, but there are a few natural alternatives depending on what exact nuance you want.