Definite article for contextually clear references

Question

Do we use "the" if we understood from the context that a noun is specific? If your answer is Yes, and I cannot imagine another answer, then the above statement contradicts with the following example which is extracted from the Guardian:

Arriving in Plymouth all passengers were invited to take a leaflet to claim compensation. I did so by post. A month later, I called customer service to follow up my claim and.......

Although that previous paragraphs imply that the writer is specifically referring to the customer service of GWR, in the second paragraph, the speaker simply said "I called customer service" (without the). Obviously this is not a typo, I found several examples like this one. So either I don't fully understand the rules of the definite article or this is another special case.

Also this one

The regulator also banned Solarplicity from taking on new customers because of poor customer service and problems with customer switching.

If I was the writer, I would write "of the poor customer service" because it is very obvious that we are talking about the customer service of Solarplicity.

Also I found the following examples, and I cannot explain why the writer used "the" or "zero article" here. See below

Perez alleges that her former boss, the US head of sales Brian Berner, took only male employees to the Sundance independent film festival in 2016 and 17. She says employees spoke of “drug use” on these “boys’ trips” and alleges that one man got into a physical fight during one. She claims these trips excluded more senior women.

I would write “the US head of the sales” because we talk about a specific sales department. It is the sales department of Spotify US. Why the writer did not use "the"?

Perez’s suit also alleges that men in the sales department received “higher compensation and equity” than their female counterparts. She quotes the company’s chief financial officer as having said that he did not care about diversity at the company and that an HR executive told staffers his favourite swear word was “cunt”.

The writer finally did what I expect, he wrote “the” sales department. I guess because we are talking about the specific sales department of Spotify.

Answer 1

... I called customer service to follow up my claim and ...

"customer service" here is a department or section of an organization, in the example probably of the railroad. In many ways it is equivalent to "the complaint department", but grammatically similar to "I called reception" or 'I called student affairs" or "I called taxpayer assistance". None of these take an article, while if they are treated as modifiers an article is normal: "I called the reception desk.", "I called the student affairs office.", "I called the taxpayer assistance hotline."

However in the later quote:

... because of poor customer service ...

"customer service" is an aspect or property of the organization, not a department. The organization may or may not have had a "customer service" department, but it did not deliver good customer service. This could be recast as

... because of the poor customer service it had shown ...

and in that construction an article would be needed. But this once standard form is now considered a bit wordy and over-formal.

This use of "customer service" is analogous to

• ... because of poor design ...
• ... because of poor implementation ...
• ... because of poor color choice ...
• ... because of poor organization ...

None of these take an article, although each could be recast in a form which does use an article All of these are natural expressions.

In the quote:

... her former boss, the US head of sales Brian Berner

"US head of sales" is the title of a position, and "US head of the sales" would be both incorrect and sound very unnatural.

In short, the use of an article does not depend solely on whether the referent is specific or not, but on what kind of construction is being used, and to a sizable extent, on the specific habit of the particular phrase. Articles are often elided, and the elided forms may become fixed phrases or at least phrases in common use.