Раскрыть скобкиPeter...(talk) on the phone when I came

Peter was talking on the phone when I came. — Когда я пришел, Питер разговаривал по телефону.
Peter wasn’t talking on the phone when I came.
Was Peter talking on the phone when I came?
What was Peter doing when I came?
Who was talking on the phone when I came?

Was Peter talking on the phone or sleeping when I came?

Peter was talking on the phone when I came, wasn't he?

Past Continuous выражает действие, на фоне которого произошло другое действие, которое обязательно должно быть выражено Past Simple.
В Past Continuous действие носит длительный характер, а в Past Simple — краткий характер.