Learn Hardcore Korean: jigeum geuphaeseo najunge jeonhwahalgeyo.

The reason clause with -아서/어서 doesn’t take past tense: say 급해서, not “급했어서.”
It’s natural for explaining the speaker’s own action, as in this sentence.

Why is 전화할게요 used instead of 전화할 거예요?

-ㄹ게요 expresses a spontaneous decision/promise made with the listener in mind. It often sounds like “I’ll do that (for you/then).”

전화할게요 = “I’ll call (you) [I promise].”
전화할 거예요 = “I’m going to call (later)” (more neutral plan/forecast, less listener-oriented).

Also, -ㄹ게요 is only used with first-person subjects (I/we).

Is this sentence polite enough? What are casual or more formal versions?

Yes—…할게요 is polite (요-form). Variants:

Casual to friends: 지금 급해서 나중에 전화할게.
More formal: 지금 급해서 나중에 전화하겠습니다.
Formal + honorific (to seniors/clients): 지금 급해서 나중에 전화드리겠습니다.

When should I use 전화드리다 instead of 전화하다?

Use 전화드리다 when the person you call deserves honorifics (older person, customer, professor, etc.). It literally means “to give a call (to you)” with respect.

Example: 나중에 선생님께 전화드릴게요.

Where is the “you” in “I’ll call you”? Do I need particles?

Korean often omits obvious participants. If you want to say it explicitly:

Informal recipient: … 너한테 전화할게.
Neutral: … 너에게/친구에게 전화할게.
Honorific recipient: … 선생님께 전화드릴게요.

What’s the difference between 나중에 and 이따가 for “later”?

나중에: later in general (unspecified future; could be much later).
이따가: a bit later, usually later the same day. In phone contexts, 이따가 is common if you mean “later today.”

Can I say 지금 바빠서 instead of 지금 급해서?

Yes, but the nuance differs:

바쁘다 = “busy” (lots to do).
급하다 = “urgent/in a rush” (time-pressed, something urgent came up). Choose based on whether you mean “I’m busy” vs “I’m in a hurry/it’s urgent.”

Could I use -니까 instead of -아서/어서 (e.g., 지금 급하니까…)?

Yes. Both work:

… 급해서 …: softer, explanatory “because/so.”
… 급하니까 …: slightly firmer/justifying “since/as.” If you’re giving a reason for asking/commanding the listener to do something, -니까 is usually preferred. Here, since it’s your own promise (“I’ll call”), -해서 is very natural.

Is the word order fixed? Can I split it into two sentences?

The reason clause usually comes first, but you have options:

지금 급해서 나중에 전화할게요. (standard)
지금 급해서요. 나중에 전화할게요. (very natural in speech: give reason, then promise)
나중에 전화할게요. 지금 급해서요. (also acceptable)

How do I say “I’ll call you back,” not just “I’ll call later”?

Add 다시 (“again/back”):

나중에 다시 전화할게요.
More polite/honorific: 나중에 다시 전화드릴게요.

Is there a difference between 전화하다 and 전화를 하다?

Both are fine and mean “to make a phone call.” In conversation 전화하다 (verb form) is a bit leaner; 전화를 하다 (noun + 하다) is also common. In honorific style you’ll often hear 전화(를) 드리다.

Any common spelling gotchas with 전화할게요?

Don’t write 할께요. The correct spelling is 할게요 (with 게). Learners sometimes over-fortify to 께, but that’s incorrect orthography.

How is the sentence pronounced naturally?

급해서 often sounds like [그패서] because ㅂ + ㅎ makes a [ㅍ] sound.
전화할게요 is [전화할게요]; many speakers pronounce the 게 a bit tense, so you may hear it like [할께요], even though it’s spelled 게.
A smooth reading: [지금 그패서 나중에 전화할게요].

Can I soften the tone further?

Yes. Add softeners like 좀 or -요 where possible:

지금 좀 급해서요. 나중에 전화드릴게요.
지금 급한 일이 있어서요. 이따가 다시 전화드릴게요.

Is 급해서 always about me being in a hurry?

Not necessarily. 급하다 can describe an urgent situation or the speaker being rushed. In this phone context, it’s naturally understood as “I’m in a hurry” or “something urgent is up (for me).” If you want to be explicit: 지금 제가 급해서… or more natural: 지금 급한 일이 있어서…