Learn Hardcore Spanish (Spain): Tengo una llamada perdida de mi jefa; le devolveré la llamada ahora. - I have a missed call from my boss; I will return the call to her now.

Because le is the indirect object pronoun (to/for her), referring to a mi jefa. The direct object here is la llamada, which is expressed with the noun itself (and could be replaced by la if needed). If you replace both objects with pronouns, you must say se la devolveré ahora (not le la), since Spanish changes le + la to se + la.

Could I say voy a devolver instead of devolveré? What’s the difference?

Yes:

Le devolveré la llamada: simple future; often used for a decision made on the spot, a promise, or a slightly more formal tone.
Le voy a devolver la llamada: periphrastic future; common and neutral, slightly more immediate or conversational.
In very immediate contexts, Spaniards also use the present: Ahora le devuelvo la llamada (“I’ll return the call now”).

Why tengo una llamada perdida and not he perdido una llamada?

They highlight different things:

Tengo una llamada perdida: focuses on the current result/state (there’s a missed call in your call log).
He perdido una llamada / Perdí una llamada: focuses on the past event of missing it.
Colloquial in Spain: Se me ha pasado una llamada (“a call slipped by me”).

What’s the point of the semicolon here? Could I use a period or a comma?

A semicolon neatly links two related independent clauses. You could also use a period: Tengo una llamada perdida de mi jefa. Le devolveré la llamada ahora. A comma alone between two full clauses is not standard punctuation unless you add a connector (e.g., así que, y).

Is llamada perdida the set phrase for “missed call”? Why is the adjective after the noun?

Yes, llamada perdida is the standard collocation for “missed call.” In Spanish, descriptive adjectives usually follow the noun. Note agreement:

una llamada perdida
dos llamadas perdidas

Where can I place the pronouns?

With a finite verb: before it → Le devolveré la llamada ahora.
With an infinitive/gerund/affirmative command: attached → Voy a devolverle la llamada ahora / Estoy devolviéndole la llamada / Devuélvele la llamada.
If you replace both objects: Se la devolveré ahora; Voy a devolvérsela ahora; Devuélvesela ya.

Can I drop la llamada and just say Le devolveré ahora?

That’s not idiomatic here; it feels incomplete because devolver normally needs a direct object (“return what?”). You can either keep the noun (le devolveré la llamada) or replace it with a pronoun (se la devolveré). Alternatively, switch to the verb llamar: La llamo ahora.

Is it also correct to say La llamo ahora or La volveré a llamar?

Yes. These are very common:

La llamo ahora / Ahora la llamo (neutral, everyday).
La volveré a llamar or Voy a volver a llamarla (I’ll call her again). Note: In Spain, many speakers say Le llamo for “I call him” (accepted leísmo with masculine), but for a woman, standard is La llamo.

Why de in una llamada … de mi jefa? Could I use por or de parte de?

For “a call from X,” Spanish uses de: una llamada de mi jefa.

por would be odd here.
de parte de is used when identifying the caller to a third party: ¿De parte de quién? — De parte de Marta.

Is jefa standard and respectful? Why not jefe?

Jefa is the regular feminine form of jefe and is perfectly standard and respectful. Use jefa if your boss is a woman; jefe if a man.

Why una and not la in una llamada perdida?

Una is indefinite—just one missed call among possibly many. La llamada perdida would point to a specific, already identified call. Also, in the fixed phrase devolver la llamada, la is definite because it refers to “the call in question” (the one you’re returning), not an arbitrary one.

Does mi need an accent, like mí?

No. Mi (my) never has an accent. Mí (me) with an accent is a stressed pronoun used after prepositions: para mí, de mí. Here it’s a possessive: mi jefa.

Could I attach pronouns to devolveré (e.g., devolveréle)?

No. Clitic pronouns attach only to infinitives, gerunds, and affirmative imperatives. With simple future, place them before the verb: Le devolveré la llamada. If using an infinitive: Voy a devolverle la llamada.

Is Le la devolveré ever correct?

No. When le/les combines with a third-person direct object pronoun (lo/la/los/las), le/les becomes se. Correct: Se la devolveré ahora.

Do I need to say Yo le devolveré…?

No. Spanish usually drops subject pronouns unless you want emphasis or contrast. Le devolveré la llamada already implies “I.”

Are there more natural alternatives in Spain for “right now”?

Yes:

ahora mismo (right this very moment)
enseguida (right away)
ya (now/already; context decides).
Examples: Ahora mismo la llamo. / Se la devuelvo enseguida. / Ya la llamo.

Pronunciation tips for Spain?

llamada: usually pronounced with yeísmo → like “ya-MA-da.”
jefa: initial j is a voiceless velar fricative → “HEH-fa” (harsh h).
devolveré: stress the final syllable “-ré” and note the acute accent → de-vol-ve-RÉ.