yoru osoku denwa ga natta node, sukosi odorokimasita.

Because 遅く is the adverb form of the い-adjective 遅い.

• 遅い = late
• 遅く = late / in a late way

In 夜遅く, the adjective is modifying the time expression, so it works like late at night.

You can think of it as:

• 夜遅い = night is late / a late night-like description
• 夜遅く = late at night

This adverbial use is very common:

• 朝早く起きました = I got up early in the morning
• 夜遅く帰りました = I came home late at night

In Japanese, some time words can be used without a particle, especially in common expressions.

So 夜遅く is a natural set phrase meaning late at night.

You could see other time expressions like:

• 昨日行きました = I went yesterday
• 今朝食べました = I ate this morning
• 夜電話しました = I called at night

If you added a particle like に, it would sound less natural here:

• 夜に遅く is not how this idea is normally expressed.

So 夜遅く should be learned as a natural time expression.

Because 鳴る is an intransitive verb.

That means the thing that makes the sound is marked with が, not を.

• 電話が鳴る = the phone rings
• ベルが鳴る = the bell rings
• アラームが鳴る = the alarm rings

So in this sentence:

• 電話が鳴った = the phone rang

Using を would be wrong with 鳴る.

This is a very common pair:

• 鳴る = to ring / to sound
  
  • intransitive
  • something rings by itself
• 鳴らす = to ring / to sound something
  
  • transitive
  • someone makes something ring

Examples:

• 電話が鳴った = The phone rang.
• ベルを鳴らした = Someone rang the bell.

So in your sentence, the phone itself is the thing making the sound, so 鳴った is correct.

Both ので and から can mean because, but they have different tones.

• ので sounds a bit softer, more explanatory, and often more objective
• から can sound more direct or more like the speaker’s personal reasoning

In this sentence:

• 電話が鳴ったので、少し驚きました。 = Because the phone rang, I was a little surprised.

This sounds natural and gentle.
If you used から, it would still be grammatical:

• 電話が鳴ったから、少し驚きました。

But ので fits well with calm explanation.

Because ので connects to the plain form of verbs.

Here:

• dictionary form: 鳴る
• past plain form: 鳴った
• reason clause: 鳴ったので = because it rang

So the structure is:

• [reason] + ので
• 電話が鳴ったので = because the phone rang

This is very normal Japanese grammar.

少し is an adverb here, meaning a little or slightly.

It modifies 驚きました:

• 少し驚きました = I was a little surprised

This is similar to English adverbs:

• 少し疲れました = I got a little tired
• 少し分かります = I understand a little

So 少し tells you the degree of surprise.

Because this sentence is written in the polite style.

• 驚いた = plain/casual
• 驚きました = polite

The rest of the sentence matches that style:

• 鳴った is in plain form because it comes before ので
• the main sentence ends politely with 驚きました

This mix is normal in Japanese:

• subordinate clause: plain form
• sentence ending: polite form

So:

• 電話が鳴ったので、少し驚きました。 is perfectly natural.

It is omitted, which is very common in Japanese.

English usually needs:

• I was surprised

Japanese often leaves out the subject when it is obvious from context:

• 少し驚きました literally just says was a little surprised

The listener naturally understands that the speaker is the one who was surprised.

So the hidden subject is basically:

• 私は or わたしは but it does not need to be said.

Yes. It is best to understand 夜遅く as a natural expression meaning:

• late at night
• late in the evening/night

It is not really being built word-by-word in the same way English learners may expect. Japanese often uses these compact time expressions:

• 朝早く = early in the morning
• 夜遅く = late at night

So when you see 夜遅く電話が鳴った, it is helpful to read it as:

• the phone rang late at night

The comma is there to separate the reason clause from the main result clause.

• 電話が鳴ったので、 = because the phone rang,
• 少し驚きました。 = I was a little surprised.

Japanese commas are somewhat flexible, but this comma helps readability and makes the sentence easier to process.

It is especially common when:

• the first clause is a reason/explanation
• the sentence has a natural pause

So the comma is not the most important grammar point, but it is very natural punctuation here.