Learn Hardcore Korean: jeonhwahal ttaemada moksoriga jakge deullyeo.

-할 때 means when (someone) does (something).
-할 때마다 means whenever/every time (someone) does (something), emphasizing repetition.
Examples:
- 비가 올 때 = when it rains
- 비가 올 때마다 우산을 써요 = whenever it rains, I use an umbrella
- 만날 때마다 반가워요 = I’m happy every time we meet
- With time nouns: 주말마다 운동해요 = I work out every weekend

Who is the subject here? Whose voice is it?

Korean often drops obvious subjects. In a phone context, 목소리 is usually understood as your voice (the person you’re talking to). If you want to say it explicitly:

Casual: 네 목소리가 작게 들려 (your voice sounds quiet)
Polite: 네 목소리가 작게 들려요 You can also add the listener marker:
너한테 전화할 때마다 네 목소리가 작게 들려요

Why is it 작게 들려 and not 작아?

N이/가 A-게 들리다 means “N sounds A (to me).” The -게 makes an adjective adverbial to describe how it is heard.
목소리가 작아 states an objective property (“the voice is quiet”).
목소리가 작게 들려 focuses on your perception (“the voice sounds quiet to me”), which is natural for phone audio.
Parallel patterns: 크게 들려 (sounds loud), 이상하게 들려 (sounds strange).

How does 들리다 differ from 듣다?

들리다 = be heard, sound (passive/intransitive). The sound is the subject with 이/가. The hearer can be marked with 에게/한테 (는): 저한테는 잘 안 들려요 (to me, it’s hard to hear).
듣다 = hear/listen (active/transitive). The listener is the subject, the thing heard takes 을/를: 저는 당신 목소리를 잘 못 들어요.
For audibility/natural descriptions of how something sounds, Koreans prefer 들리다: 네 목소리가 잘 안 들려요 is more idiomatic than 네 목소리를 잘 못 들어요 in this context.

Can I say 작아 들려 or 작은 들려?

No. With 들리다, use the adverbial -게 form:

Correct: 작게 들려요, 크게 들려요, 이상하게 들려요
Incorrect: 작아 들려요, 작은 들려요, 큰 들려요

Is 낮게 들려 okay? Does it mean the same as 작게 들려?

Different meaning:

낮게 (from 낮다) refers to low pitch.
작게 (from 작다) refers to low volume (quietness). So 목소리가 낮게 들려요 = your voice sounds low-pitched; 목소리가 작게 들려요 = your voice sounds quiet.

Why is it 목소리가 and not 목소리는?

이/가 marks neutral/new information in descriptive statements: 목소리가 작게 들려요.
는 sets up a topic/contrast: 목소리는 작게 들리는데 배경 소음은 크게 들려요 (as for the voice, it sounds quiet, but the background noise sounds loud).

Can I drop the particle 가 in casual speech?

Yes. In informal conversation, particle drop is common:

목소리 작게 들려 (casual and natural)
In writing or formal speech, keep the particle: 목소리가 작게 들립니다.

What are polite or formal versions of the sentence?

Polite: 전화할 때마다 목소리가 작게 들려요.
Formal: 전화할 때마다 목소리가 작게 들립니다.
Softer/more polite: 죄송한데, 통화할 때마다 목소리가 조금 작게 들려요.

Can I rearrange the word order?

Some flexibility is fine:

전화할 때마다 목소리가 작게 들려요 (most natural)
목소리가 전화할 때마다 작게 들려요 (okay, slightly heavier) Avoid splitting the predicate unnaturally (e.g., 전화할 때마다 작게 목소리가 들려요 sounds awkward). Time/frequency phrases like 때마다 usually go early in the sentence.

Should it be 전화하다 or 통화하다?

전화하다 = to place a call (“call someone”).
통화하다 = to be on the phone / have a call. If you mean “whenever we’re on the phone,” 통화할 때마다 is often more precise:
통화할 때마다 목소리가 작게 들려요. If you mean “whenever I call you,” specify the person:
너한테 전화할 때마다 목소리가 작게 들려요.

Why is it spaced 전화할 때마다 and not 전화할때마다?

-ㄹ/을 때 uses 때, a dependent noun, so there’s a space before 때: 전화할 때.
마다 is a particle that attaches to the noun without a space: 때마다. Hence: 전화할 때마다 (correct), not 전화할때마다.

Is 전화를 할 때마다 also correct?

Yes. 전화하다 and 전화를 하다 are both used. You can say:

전화할 때마다 … (more compact)
전화를 할 때마다 … (also natural) Similarly, 통화를 할 때마다 … is fine.

Can I use 말 instead of 목소리? For example, 말이 작게 들려?

말이 잘 안 들려요 (I can’t hear/catch what you’re saying) is common.
말이 작게 들려요 sounds odd; 작다 (small/quiet in volume) collocates with the sound itself: 목소리 or 말소리.
Natural: 목소리가/말소리가 작게 들려요.

Any pronunciation tips for this sentence?

작게 often surfaces as [작께] (tensification of ㄱ).
In 전화할 때마다, say a clear space between 할 and 때; it flows smoothly but keep the syllables distinct.
들려 is pronounced [들려] (no extra vowel between ㄹ and ㅕ).

Why is it 전화할 때 (with -ㄹ)—does that mean future?

No. The -ㄹ/을 before 때 is the adnominal modifier used to form “when” with actions. It doesn’t inherently mark future; it’s neutral about time unless context or tense elsewhere sets it.

Past reference: 전화했을 때 (when I called/when you called).
Habitual/general: 전화할 때 (when(ever) I/you call).