Wir buchen mehr Datenvolumen zu.

Because the verb here is zubuchen, which is a separable verb.

In a normal main clause, separable verbs split:

• Wir buchen mehr Datenvolumen zu.

But in the infinitive, they stay together:

• mehr Datenvolumen zubuchen

And with a modal verb:

• Wir wollen mehr Datenvolumen zubuchen.

So the zu at the end is not the infinitive marker to. It is the separable prefix belonging to buchen.

On its own, buchen often means to book, to reserve, or in some contexts to charge/post.

With the prefix zu-, zubuchen means something like:

• to add on
• to book additionally
• to top up with an extra package

In mobile phone/internet contexts, Datenvolumen zubuchen is a common way to say that extra data is added to an existing plan.

So zubuchen is not just book in the travel sense. It means adding something extra to a service or contract.

Because German very often forms compound nouns by combining words into one.

Here:

• Daten = data
• Volumen = volume

Together:

• Datenvolumen = data volume, meaning your data allowance/data amount

This is completely normal in German. English often writes such things as two words, while German usually combines them into one noun.

German often leaves out the article when talking about an unspecified amount of something, especially with quantity words like mehr.

So:

• mehr Datenvolumen = more data allowance / more data

This works much like English more data, where you also would not normally say more the data.

You could use an article in other contexts, but here it would sound less natural:

• Wir buchen mehr Datenvolumen zu. = natural
• Wir buchen ein höheres Datenvolumen zu. = possible, but more specific/formal in tone

Because Datenvolumen is treated here like an uncountable quantity, similar to English data, water, or time.

• mehr = more
• mehrere = several

You use mehrere with countable items:

• mehrere Tarife = several plans
• mehrere Optionen = several options

But Datenvolumen is not normally counted as separate units in this sentence. It is a quantity, so mehr is the right choice:

• mehr Datenvolumen = more data

It is the accusative, because it is the direct object of zubuchen.

The structure is:

• Wir = subject
• buchen ... zu = verb
• mehr Datenvolumen = thing being added/booked

So the sentence answers What are we adding?

• mehr Datenvolumen

That makes it the direct object.

You do not see a clear case ending here because:

• Datenvolumen is neuter
• with no article, the noun itself does not visibly change here

Yes. It is in the present tense.

• wir buchen = we book / we are booking / we add

As in German generally, the present tense can cover both a simple present and a present-progressive kind of meaning, depending on context.

So this could mean:

• We add more data
• We’re adding more data
• We book extra data onto the plan

Yes, especially in a telecom or customer-service context.

It sounds like something a mobile provider, app, or support page might say when talking about adding extra data to a contract.

Depending on context, Germans might also say things like:

• Wir buchen zusätzliches Datenvolumen dazu.
• Wir buchen mehr Datenvolumen auf.
• Wir fügen mehr Datenvolumen hinzu.

But Datenvolumen zubuchen is a very common and natural expression in this specific context.

No, not in a normal main clause.

Because zubuchen is separable, the correct main-clause pattern is:

• Wir buchen mehr Datenvolumen zu.

You keep it together only in forms like the infinitive or with another verb:

• mehr Datenvolumen zubuchen
• Wir möchten mehr Datenvolumen zubuchen.

So Wir zubuchen mehr Datenvolumen is not standard German.

The basic pattern is:

• Wir = subject
• buchen = conjugated verb in second position
• mehr Datenvolumen = object
• zu = separable prefix at the end

So it follows the normal German main-clause rule that the finite verb comes second.

That is why you get:

• Wir buchen mehr Datenvolumen zu.

and not:

• Wir mehr Datenvolumen buchen zu.

Yes. In the infinitive, the verb is written together:

• mehr Datenvolumen zubuchen

Examples:

• Wir wollen mehr Datenvolumen zubuchen.
• Es ist möglich, mehr Datenvolumen zuzubuchen.

Notice the last example:

• zuzubuchen

That happens because:

• the verb is zubuchen
• and the infinitive marker is also zu

So in some structures, both appear:

• zu + zubuchen = zuzubuchen

This often surprises learners, but it is completely normal.