"We are calling our grandson."
Translation:Nous sommes en train d'appeler notre petit-fils.
Yes but I think this exercise is in a new skill concerning the "present progressives" to teach how to express that you are in the process right now of doing something. It is like saying "We are in the middle of calling our grandson" in English to indicate that you can't be interrupted.
Frustrating question. "en train" is brand new to me. It would be nice if Duolingo prepped you for this form of present progressives. What does "en train" mean???
The topic of continuous tense can be found in the Tips and Notes section of the Basics 3 Unit, Je suis en train de faire (quelque chose) - I am in the process of doing (something). For example "Je suis en train de manger." I am in the process of eating.