I was studying when the phone rang , seems not to be continuous as it refers to a moment in time when the phone rang. Your examples have "used to " ?

Hola David

This sentence should be: "Estaba estudiando/Estudiaba cuando sonó el teléfono."

The first part is expressing a continuous action "estaba estudiando or estudiaba" and the second part shows a sudden action "sonó el teléfono". It is common to have this combination of tenses. Can you tell us which lesson you are referring to?

Gracias 

Inma