Angenommen, dass der Ansprechpartner fehlt, senden wir eine Raumanfrage.

Not really. This is the first thing many learners would notice.

In German, angenommen can be used to mean assuming or suppose, but it is usually followed directly by a clause, without dass:

• Angenommen, der Ansprechpartner fehlt, senden wir eine Raumanfrage.

If you want to keep dass, a more natural structure would be something like:

• Unter der Annahme, dass der Ansprechpartner fehlt, senden wir eine Raumanfrage.

In everyday German, many speakers would probably choose:

• Wenn der Ansprechpartner fehlt, senden wir eine Raumanfrage.
• Falls der Ansprechpartner fehlt, senden wir eine Raumanfrage.

So the sentence is understandable, but Angenommen, dass ... is not the most idiomatic wording.

Here, angenommen means assuming, supposing, or let’s assume.

It comes from the verb annehmen, which normally means to assume, to accept, or to take depending on context. In this sentence, it is being used almost like a linking word to introduce a condition.

So:

• Angenommen, ... = Assuming that ... / Suppose that ...

This is different from its literal past participle use, such as:

• Die Einladung wurde angenommen.
  = The invitation was accepted.

Because dass introduces a subordinate clause, and in German subordinate clauses the finite verb usually goes to the end.

So:

• dass der Ansprechpartner fehlt

has this order:

• dass = that
• der Ansprechpartner = the contact person
• fehlt = is absent / is missing

This verb-final pattern is very common after words like:

• dass
• weil
• wenn
• falls
• obwohl

For example:

• Ich weiß, dass er kommt.
• Wenn er fehlt, senden wir eine Anfrage.

Because German is a verb-second language in main clauses.

The whole introductory part comes first:

• Angenommen, dass der Ansprechpartner fehlt,

That entire clause counts as position 1. The finite verb of the main clause must then come in position 2:

• senden wir eine Raumanfrage

So the pattern is:

• first element: Angenommen, dass der Ansprechpartner fehlt
• second element: senden
• then the subject: wir

This is the same pattern as in:

• Heute senden wir eine Raumanfrage.
• Wenn er fehlt, senden wir eine Raumanfrage.

If there were no introductory clause, you would say:

• Wir senden eine Raumanfrage.

Because German uses a comma to separate a subordinate or introductory clause from the main clause.

So the structure is:

• introductory clause: Angenommen, dass der Ansprechpartner fehlt
• main clause: senden wir eine Raumanfrage

German comma rules are stricter than English ones in this area. Clauses like this normally must be separated by a comma.

Ansprechpartner is a masculine noun, so in the nominative singular it takes der:

• der Ansprechpartner

In this sentence, it is the subject of fehlt, so nominative is the correct case.

Also, the word means contact person or point of contact.

A learner may also wonder whether this has to refer to a male person. Grammatically, Ansprechpartner is masculine. In modern usage, depending on style and context, you may also see:

• die Ansprechpartnerin = female contact person
• die Ansprechperson = gender-neutral alternative

Fehlt comes from fehlen.

Here it means something like:

• is absent
• is missing
• is not available

That can be tricky for English speakers, because German fehlen does not work exactly like English to miss.

For example:

• Der Ansprechpartner fehlt.
  = The contact person is absent / missing.

But:

• Ich vermisse ihn.
  = I miss him.

So in your sentence, fehlt is about absence, not about emotional missing.

Raumanfrage is a compound noun:

• Raum = room
• Anfrage = inquiry, request

So Raumanfrage means something like:

• room request
• inquiry about a room
• request for a room

German often creates nouns by combining smaller nouns into one longer word. The final part determines the grammatical gender.

Since Anfrage is feminine:

• die Anfrage
• die Raumanfrage

That is why the sentence has eine Raumanfrage.

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

The verb senden here means to send, and what is being sent?

• eine Raumanfrage

So:

• wir senden eine Raumanfrage

Because Raumanfrage is feminine, the indefinite article is:

• nominative: eine
• accusative: eine

So the form does not change, even though the case is accusative.

Yes, very likely.

If the idea is simply if the contact person is absent, we send a room request, then German speakers would often prefer:

• Wenn der Ansprechpartner fehlt, senden wir eine Raumanfrage.
• Falls der Ansprechpartner fehlt, senden wir eine Raumanfrage.

The difference is roughly:

• wenn = if / whenever
• falls = if, in case it happens
• angenommen = assuming, suppose

So angenommen sounds more like a hypothetical setup, while wenn or falls sounds more natural for an ordinary condition.