When I called her, she was studying.
I know what it means.
When I called her, she studied.
But does the second sentence mean she started studying when I called her?
The past tense would not often be used in that sentence. It suggests that the act of "studying" happened completely in the past at the time when you called her and perhaps as a result of calling her. Consider
When I shouted her name, she sat up.
The action of "sitting up" happened at the time of shouting her name, and as a result of the shout.
On the other hand, studying usually takes a long time. It takes longer to study than it takes to call someone, so "When I called her, she studied" is grammatically okay, but not a likely or meaningful sentence of English.