Questions & Answers about Они проговорили по телефону целый час.
Why is проговорили perfective if "talking for an hour" describes an ongoing process?
This is the exact purpose of the про- prefix when used for duration. While говорить (to speak/talk) focuses on the ongoing process, adding про- creates a perfective verb that means "to spend a specific amount of time doing an action." It treats that entire block of time as a single, completed event.
Does Russian not use a preposition to mean "for" in "for a whole hour"?
That is correct. To express how long an action lasted, you simply use the accusative case with no preposition. Here, целый час is the accusative of time. Because час is a masculine inanimate noun, its accusative form looks identical to the nominative case.
Why do we use the preposition по for "on the phone", and what case follows it?
The preposition по requires the dative case here, which gives us телефону. In Russian, по + dative is the standard way to express a medium of communication or broadcasting. You will also see this in phrases like по телевизору (on TV), по интернету (on the internet), or по радио (on the radio).
How would the nuance change if we used the imperfective говорили instead of проговорили?