tuuwameisai ga hukuzatuna ue ni, goseikyuu mo aru.

A natural reading is:

つうわめいさい が ふくざつな うえに、ごせいきゅう も ある。

Word readings:

• 通話明細 = つうわめいさい
• 複雑 = ふくざつ
• 誤請求 = ごせいきゅう

うえに means on top of that, in addition, or what is more.

It is used to add another point, often to show that one bad thing is followed by another bad thing, or one good thing by another good thing.

So the structure here is basically:

• The call details are complicated
• and on top of that, there are also incorrect charges

In this sentence, うえに has a slightly formal or written feel compared with something simpler like し.

Because 複雑 is a na-adjective.

When a na-adjective directly modifies a noun-like expression, it usually takes な. In this grammar pattern, うえ behaves like a noun, so you say:

• 複雑なうえに

Compare:

• 静か → 静かな部屋
• 複雑 → 複雑なうえに

This is the same reason you get 便利なうえに, 大変なうえに, and so on.

Here, が marks 通話明細 as the subject of 複雑な.

So the first part is:

• 通話明細が複雑な = the call details are complicated

You could think of it as identifying what has the property of being complicated.

In some contexts, learners wonder whether は could appear instead. It could, if you wanted more of a topic-comment feel:

• 通話明細は複雑なうえに、誤請求もある。

That would sound like:

• As for the call details, they are complicated, and on top of that, there are also incorrect charges.

So が is not strange here; it is just a straightforward subject marker.

も means also or too.

It shows that incorrect charges are an additional problem.

Without も:

• 誤請求がある = there are incorrect charges

With も:

• 誤請求もある = there are also incorrect charges

That matches the additive feeling of うえに very well. The sentence is piling up complaints:

• The details are complicated,
• and on top of that,
• there are also billing errors.

ある is used for the existence of things that are not living beings.

Here, 誤請求 is a thing or issue, not a person or animal, so Japanese uses ある:

• 誤請求がある = there are incorrect charges

You would not use いる here, because いる is for animate beings.

No. うえに can be used for both positive and negative additions.

Examples:

• この店は安いうえに、おいしい。
  The restaurant is cheap, and on top of that, delicious.
• この機種は高いうえに、使いにくい。
  This model is expensive, and on top of that, hard to use.

In your sentence, it happens to connect two negative points, so it sounds like a complaint.

The basic patterns are:

• Verb (plain form) + うえに
  • 時間がかかるうえに、お金もかかる。
• i-adjective + うえに
  • 値段が高いうえに、質も悪い。
• na-adjective + な + うえに
  • 手続きが複雑なうえに、時間もかかる。
• Noun + の + うえに
  • 雨のうえに、風も強い。

Your sentence uses the na-adjective pattern:

• 複雑なうえに

Yes. A more casual alternative would be:

• 通話明細が複雑だし、誤請求もある。

Here, し also means something like and, plus, or besides.

Difference in nuance:

• うえに sounds a bit more formal, structured, or written.
• し sounds more conversational.

Both are natural, but うえに fits well in customer-service, complaint, or explanatory writing.

通話明細 refers to a call statement, call details, or itemized call record.

It is the kind of list that shows phone calls, times, durations, charges, and similar information.

So in this sentence, the complaint is not about the phone calls themselves, but about the billing/details statement being hard to understand.

誤請求 means incorrect billing, billing errors, or wrong charges.

It is made from:

• 誤 = mistake, error
• 請求 = claim, charge, billing request

So 誤請求もある means that the bill does not just look complicated; it also contains actual mistakes.

The comma is helpful, but not absolutely required.

• 通話明細が複雑なうえに、誤請求もある。
• 通話明細が複雑なうえに誤請求もある。

Both are understandable. The comma just makes the pause clearer and improves readability, especially in written Japanese.

They are similar, but not exactly identical in usage.

• うえに is a grammar pattern that attaches directly to the preceding phrase.
• その上 is more like an independent adverb meaning moreover or besides that.

For example:

• 通話明細が複雑なうえに、誤請求もある。
• 通話明細は複雑だ。その上、誤請求もある。

These are very close in meaning. The first is more tightly connected as one sentence; the second breaks it into two parts a bit more explicitly.