Téléphonais-tu souvent à tes cousins quand tu étais petit?
Translation:
Did you call your cousins often when you were little?
The options:
Nous parlions au téléphone tous les samedis. - "We talked on the phone every Saturday."
J'ai téléphoné samedi. - "I phoned Saturday."
Je vais téléphoner samedi. - "I will call Saturday."
The question requires an answer conveying a continual action ("did you repeatedly call your cousins?"); the second option speaks of an activity that you did once and never repeated, and the last option speaks of something you haven't yet done. Thus, the correct option is the first one.
Answer:
Nous parlions au téléphone tous les samedis.