Learn German: Sooft das Callcenter anrief, legte mein Vater sofort auf. - Whenever the call center called, my father hung up immediately.

Sooft is a subordinating conjunction, which kicks the conjugated verb anrief to the end of its clause. Because this entire subordinate clause comes first, it takes up position one of the whole sentence. That forces the main clause to start immediately with its verb, legte.

Could I use immer wenn here instead of sooft?

Yes, you could say immer wenn das Callcenter anrief. The conjunction sooft is just a slightly more emphatic and elegant way to express that an action happened every single time, literally translating closer to 'as often as'.

Why are anrief and legte in the simple past (Präteritum) if this was a repeated action?

In German, the simple past is standard for recounting past events in writing or storytelling, even for habits or repeated actions. Unlike English, which often uses 'would call' or 'used to hang up' to show repetition, German relies on temporal words like sooft to indicate that the event happened multiple times.

How does the separable verb auflegen (to hang up) behave in this past tense structure?

Just like in the present tense, the conjugated base verb (legte) takes the standard verb position for a main clause following a comma, and the separable prefix (auf) gets sent to the very end of the sentence.