Então contactei a seguradora.

Because Portuguese often drops subject pronouns when the verb ending already shows who the subject is. Contactei already means I contacted, so eu is not necessary.

You can add eu for emphasis or contrast, for example: Eu contactei a seguradora, não foi ele.

Here então links this sentence to what came before. Depending on context, it can feel like then, so, or well.

In a narrative, it often means then: one thing happened, and then you contacted the insurer. In conversation, it can also sound more like a discourse marker, similar to so.

It depends on how então is being used.

If it is a discourse marker, a comma is very common: Então, contactei a seguradora.

If it simply means then in a sequence of events, no comma is also very natural: Então contactei a seguradora.

So the version without a comma is perfectly possible, especially if the idea is then I contacted the insurer.

The stress is on the second syllable: en-TÃO.

The ending -ão is nasal, which is difficult for English speakers because there is no exact equivalent in English. Also, in European Portuguese, the first syllable is often reduced in fast speech, so então can sound shorter and more compressed than learners expect.

A rough English-friendly approximation is en-TOW, but with a nasal sound at the end rather than a clear English ow.

Contactei is the pretérito perfeito simples: the simple past / preterite.

It is the first person singular form of contactar. It is used for a completed action in the past, so it fits a sentence describing a finished step in a sequence of events.

In European Portuguese, the usual verb is contactar, so the past form is contactei.

The form contatar / contatei is mainly associated with Brazilian Portuguese. If you are learning Portuguese from Portugal, contactar is the standard form to use.

Because contactar takes a direct object in Portuguese.

So you say:

contactei a seguradora

not

contactei com a seguradora

If you want to use a different expression, then a preposition may appear, for example:

entrei em contacto com a seguradora

That expression needs com, but contactar itself does not.

Because a is the definite article, meaning the.

Portuguese uses definite articles very naturally before specific nouns, especially when the speaker is talking about a known, identifiable thing. Here, a seguradora means a specific insurer already understood from the context.

Leaving out the article would usually sound less natural in a normal full sentence.

Contactar is completely normal and natural in European Portuguese. It is neutral and very common.

It is also slightly broader than verbs like telefonar or ligar, because contactar does not say how the contact happened. It could have been by phone, email, message, and so on.

So contactei a seguradora is a very natural choice if you just want to say that you got in touch with the insurer without specifying the method.