Learn Finnish: Hän jätti minulle kiireellisen soittopyynnön, jota en huomannut heti. - He left me an urgent callback request which I did not notice immediately.

Jota refers back to the callback request (soittopyyntö). It is in the partitive case because it acts as the object of the negative verb phrase en huomannut (I did not notice). In Finnish, the object of a negative sentence is always in the partitive.

Why does kiireellisen soittopyynnön take the accusative -n ending instead of the partitive?

The affirmative verb jättää (to leave) takes a total object here because a complete, single request was left. This means the object takes the genitive-looking accusative -n ending. If the sentence were negative ('He didn't leave...'), it would be in the partitive.

Why are both verbs (jätti and en huomannut) in the simple past tense?

Even though he left the request before you failed to notice it, Finnish often uses the simple past (imperfect) for a sequence of past events where the chronological order is obvious. There is no strict sequence of tenses rule forcing the pluperfect (oli jättänyt, had left) here.

How is the word soittopyyntö constructed?

It is a compound word made of two nouns that are derived from verbs: soitto (a call, from soittaa, to call) and pyyntö (a request, from pyytää, to request).