When I talked with my grandma on the phone, she sounded weak

When I talked with my grandma on the phone, she sounded weak, but by the time we ___ up, her voice had been full of life.
A. were hanging
B. had hung
C. hung正确
D. would hang
为什么D对?D可以理解为在我们将要挂电话的时候么?
D不对，答案选C