"Nous sommes en train d'appeler notre petit-fils."
Translation:We are calling our grandson.
être en train de means "to be in the process of" - to be in the middle of doing something. It is always followed by a verb in the infinitive.
Shouldn't "trying" be an acceptable translation of "en train" in this case? "We are trying to call our grandson." "Trying" is a bit overbroad here, but it conveys the sense that the event is happening now, while making "en train" seem less useless to English speakers. Until this point in the DL course, "Nous appelons notre petit-fils" was the way to translate both "We are calling our grandson," and, "We call our grandson." For DL to suddenly offer "en train" to distinguish between the two seems arbitrary and redundant when viewed from a student's perspective. I'm in favor of using "en train" to distinguish between the two, but for it to come at this point in the course is an odd decision.
But "en train de" doesn't translate as "trying to." I don't think it's too early for students to be introduced to an alternate way of conveying the progressive. Depending on context, "Nous appelons" could be "We are calling," but now we know that you can also use "Nous en train d'appeler." You could also say "We are in the middle of calling" or "We are in the process of calling."