Escribimos nuestro teléfono en caso de que necesiten llamar.

Escribimos can mean either we write (present) or we wrote (preterite) because those two forms look the same for regular -ir verbs in the nosotros form.

So context tells you which one it is. In this sentence, en caso de que necesiten llamar points to a present/future idea, so we write / we write down is the natural reading.

In context, escribir often means to write down information.

So Escribimos nuestro teléfono does not usually mean we write our telephone in a literal awkward way. It means we write down our phone number/contact number.

Spanish often uses the simple verb where English prefers a phrasal verb like write down.

Here it most naturally means our phone number.

In Spanish, especially in everyday usage, teléfono can refer to:

• the physical phone
• the telephone number

In a sentence about giving contact information, learners should understand nuestro teléfono as our phone number.

En caso de que is a set expression meaning in case.

It introduces something that might happen, usually as a precaution:

• Lleva agua en caso de que haga calor.
• Te escribo en caso de que cambien los planes.

So here, the idea is: We write down our phone number in case they need to call.

Because en caso de que normally triggers the subjunctive.

The reason is that the action is not presented as a fact. It is only a possibility:

• maybe they will need to call
• maybe they will not

So Spanish uses:

• en caso de que necesiten llamar

not:

• en caso de que necesitan llamar

That is one of the most common subjunctive patterns to learn.

The subject is implied, not stated. Necesiten could refer to ellos, ellas, or ustedes, depending on context.

Spanish often leaves subject pronouns out because the verb ending already gives you the information. Here, necesiten is a plural form, so it means something like:

• in case they need to call
• in case you all need to call

You need the larger context to know exactly which one.

Because after necesitar, Spanish commonly uses an infinitive:

• necesitar + infinitive = to need to do something

So:

• necesiten llamar = they need to call

This is the same pattern as:

• Necesito comer. = I need to eat.
• Necesitan salir. = They need to leave.

Good question. Spanish can leave the object unstated if it is obvious from context.

Because the sentence already mentions our phone number, the listener can infer that the call would be to us. So llamar by itself works fine.

If you want to make it explicit, you could say:

• en caso de que necesiten llamarnos

That would mean in case they need to call us.

Without nos, the sentence can still sound natural because the destination of the call is understood.

Yes, but the nuance is a little different.

• en caso de que necesiten llamar = in case they need to call / just in case they need to call
• si necesitan llamar = if they need to call

En caso de que sounds more like a precaution or preparation.
Si is a more neutral conditional.

So both can work, but en caso de que better matches the idea of giving a number ahead of time as a preventive step.

Yes. Por si is another very common way to say in case, especially in everyday speech.

Compare:

• Escribimos nuestro teléfono en caso de que necesiten llamar.
• Escribimos nuestro teléfono por si necesitan llamar.

The second version is often more conversational.

A useful difference to notice:

• en caso de que
  • usually subjunctive
• por si
  • usually indicative

So:

• en caso de que necesiten
• por si necesitan

Both can be correct, but the grammar pattern changes.

Because Spanish usually uses a possessive adjective instead of an article in this kind of phrase.

So you say:

• nuestro teléfono
• mi casa
• su dirección

not usually:

• el nuestro teléfono

The possessive already does the job of identifying whose phone number it is.