Hola Miles

"Fuimos nosotros los que llamamos a la puerta." is also grammatically correct. 

The only reason why it is incorrect here is because we are testing on conjugation of ser in El Imperfecto and that is what the hint is asking.

Both with fuimos and éramos, the translation would be the same: It was us who knocked on the door / We were the ones who knocked on the door.

Fuimos would refer to an action that we see as a completed action, as finished (one of the uses of the Indefinido) and Éramos would imply more of a descriptive action where the beginning and end of that action is not relevant. 

You may find this lesson helpful as it is explaining this precisely. Have a look here.

Saludos

Inma