Learn Finnish: Hän ei tullut kokoukseen. Sen sijaan hän soitti minulle. - He did not come to the meeting. Instead, he called me.

Not effectively. The lesson's focus, taas, translates better to 'whereas' or 'on the other hand' and is typically used to contrast two different subjects. Sen sijaan translates exactly to 'instead,' showing that the second action (calling) replaced the first action (coming to the meeting) for the exact same subject.

What is the literal breakdown of the phrase sen sijaan?

It literally translates to 'into the place of it.' It is formed by sen (the genitive of se, meaning 'of it') and sijaan (the illative form of sija, meaning 'into place'). You can apply this same logic to other pronouns and nouns, such as Pekan sijaan (instead of Pekka).

Why is the word for meeting, kokous, in the illative case (kokoukseen)?

The verb tulla (to come) requires a destination case. When referring to events, gatherings, or enclosed spaces, you use the illative case (into). So in Finnish, you do not just come 'to' a meeting, you come 'into' it.

Why is the pronoun minä in the allative case (minulle) here?

When the verb soittaa is used to mean 'to call on the phone,' the person being called must be in the allative case (-lle). You are literally 'calling to' them.