КАК правильно поставить время? . Who you (talk) to on the phone as I came in? — I (talk) to Mr Pitt.

The traffic made so much noise that i couldn't hear what he HAD SAID
From the sounds it was clear that Marry WAS PRACTISING the piano.
John WAS WORKING all the day yesterday.
He watched the children for a moment. Some of them WERE BATHING in the sea, others WERE PLAYING on the sand.
It WAS RAINING when we went out.
I WAS PLAYING the piano when he came into the room.
I came into the room when Mary WAS PLAYING the piano.

Who were you talking to on the phone as I came in? — I was talking to Mr Pitt.

Who had you been talking to on the phone as I came in — I'd been talking to Mr Pitt.

Whom were you talking to when I came in?
I was talking to Mr. Pitt

Подсказка на будущее- когда два действия одновременно, как правило будет или оба Continuous, либо Continuous для действия которов уже происходило на тот момент, когда произошло второе, тогда второв, внезапное будет в Past Simple:

When I walked in the room my wife was kissing my brother.

А можно встречный вопрос: вот само задание в том виде, в которoм вы написали, откуда оно, я имею в виду:
" Who you (talk) to on the phone as I came in? "
дело в том, что там грубая ошибка, интересен источник, имя автора учебника. Спасибо :)

Who were you talking to over the phone as I came in? - I was talking to Mr.Pitt