---What____you_____(do)just now?---I____(chat)with my friend on the phone.

---What____you_____(do)just now?---I____(chat)with my friend on the phone.
---What____you_____(do)just now?---I____(chat)with my friend on the phone.

---What____you_____(do)just now?---I____(chat)with my friend on the phone.
did do
chatted

did do
was chatting

did do

what were you doing just now?
I was chattin with my friend on the phone
由于强调just now，有明确时间点，一定用过去进行时，同理，回答也应该用过去进行时，不过chat变ing形式注意双写t变成chatting
你刚刚在做什么？——我在和朋友通电话