Search:

It is a homonym. It has nothing to do with почитать 'to read for a while'.
Although they are etymologically related (I guess), it makes little sense to treat them as related in the modern language....

IMO, the perfective aspect indicates not only some result, but a transition to a new state in general.

Я приготовил ужин. --> the new state is the existence of the dinner.
Вода потекла по руслу....

Well... since the idea of "to be expected to do" is not a direct part of the grammatical meaning of the Russian aspect, there can be some patterns, but no strict rules.

As far as I can see, there...

Yep. :) Thank you.

Should not the later one be Я не звонила потому что не могла?

I don't think so. It is not about successfully completed 'not phoning'.
The perfective here is used in its usual meaning 'single action', 'action as a whole', action that has no significant time...