Mañana por la mañana volveré a llamar para ver si queda alguna hora disponible.

Because the two mañanas mean different things:

• mañana = tomorrow
• por la mañana = in the morning

So Mañana por la mañana means tomorrow morning.

This is completely normal in Spanish, even though it sounds repetitive to an English speaker.

Por la mañana is a fixed expression meaning in the morning.

Spanish commonly uses:

• por la mañana = in the morning
• por la tarde = in the afternoon / evening
• por la noche = at night / in the evening

You usually just learn these as set phrases. The por here is not something you translate word-for-word in a simple way; it is just part of the normal expression.

Literally, volveré a llamar is something like I will return to call, but the natural meaning is:

• I will call again
• I will call back

Spanish very often uses the structure:

• volver a + infinitive = to do something again

Examples:

• Volveré a escribir. = I’ll write again.
• Voy a volver a intentarlo. = I’m going to try again.

So volveré a llamar is a very common way to say I’ll call again / I’ll call back.

Because the verb pattern is:

• volver a + infinitive

So:

• volver a llamar = to call again
• volver a intentar = to try again
• volver a empezar = to start again

That a is required by the structure. You cannot say volveré llamar.

Volveré is the first-person singular simple future of volver:

• volveré = I will return / I will do again

Here it means I will call again.

Spanish often uses the simple future for plans or intentions, especially in slightly more formal or neutral speech. In everyday conversation, people also often use ir a + infinitive:

• Mañana por la mañana voy a volver a llamar...

Both are correct.
Volveré sounds a little more concise and slightly more formal or definite.

Yes. Llamaré otra vez also means I’ll call again.

Both are natural:

• Volveré a llamar
• Llamaré otra vez

A small nuance:

• volver a + infinitive is a very common Spanish structure
• otra vez directly emphasizes again

In this sentence, volveré a llamar sounds very idiomatic and natural.

Para ver si means to see if or in order to see if.

Breakdown:

• para = in order to / for
• ver = to see
• si = if / whether

So:

• para ver si queda alguna hora disponible = to see if there is any available time slot left

This is a very common Spanish expression when checking availability, possibilities, or results.

Examples:

• Te llamo para ver si estás en casa. = I’m calling to see if you’re home.
• Voy a mirar para ver si funciona. = I’m going to check to see if it works.

Because si without an accent means if.

Compare:

• si = if
• sí = yes

So in this sentence:

• para ver si queda... = to see if there remains / if there is left...

The accent changes the meaning completely.

Because its subject is alguna hora disponible, which is singular.

• alguna hora disponible = some available time / any available slot

So Spanish uses:

• queda alguna hora disponible = is there any available time left? / does any appointment slot remain?

If the subject were plural, the verb would also be plural:

• quedan algunas horas disponibles = there are some available times left

So:

• queda ↔ singular noun
• quedan ↔ plural noun

Quedar has several meanings, and here it means:

• to remain
• to be left
• to still be available

So queda alguna hora disponible means something like:

• is there any available time left?
• does any appointment slot remain?

This use of quedar is very common for availability:

• Quedan entradas. = There are tickets left.
• No queda pan. = There’s no bread left.
• ¿Queda sitio? = Is there any room left?

So yes, quedar can mean other things too, but in this sentence it means to remain / be left.

In Spanish, hora can mean a clock time, an hour, or a time slot, depending on context.

In this sentence, alguna hora disponible most naturally refers to:

• an available appointment time
• an available slot

This is especially common when talking about doctors, hairdressers, offices, reservations, and similar situations.

For example:

• ¿Le queda alguna hora para mañana? = Do you have any appointment slots left for tomorrow?

English often says time, slot, or appointment, while Spanish may simply use hora.

Alguna means some or any, depending on context.

Here it gives the sense of:

• any available time
• some available slot

So:

• alguna hora disponible = any available time slot

If you said una hora disponible, it would sound more like one available hour/time, which is less natural in this context.

After si, Spanish often uses this kind of indefinite wording:

• ver si hay alguna mesa libre
• ver si queda algún billete
• ver si tiene alguna cita

So alguna is the natural choice here.

Because they do different jobs:

• alguna is a determiner, like some / any, so it goes before the noun
• disponible is an adjective, and Spanish adjectives often come after the noun

So:

• alguna hora disponible = any available time

This is the normal word order.

Yes, that is also possible.

Spanish often uses the present tense to talk about near-future plans:

• Mañana por la mañana vuelvo a llamar...

That would sound quite natural in conversation.

The difference is small:

• volveré a llamar = I will call again
• vuelvo a llamar = I’m calling again tomorrow / I call again tomorrow

The future tense here sounds a little more explicitly future and slightly more formal or deliberate.

Yes, it sounds very natural in Spain.

A few points that fit Spain well:

• por la mañana is standard everywhere
• volver a llamar is very common
• hora disponible is a normal way to talk about appointment availability

In Spain, this would sound like a polite and ordinary sentence you might say when calling a clinic, salon, office, or similar place.