Vous a-t-il appelé

Bonjour Tom,

Thank you for pointing this typo out. It has now been corrected.

Bonne journée

Bonjour Kathleen,

L'Imparfait is used here because it is expressing a continuing action and giving a description (see link here) :

qui venait du téléphone fixe = which came (was coming) from the landline -> ongoing

qui est venu du téléphone = which has come from the landline -> completed

I hope this is helpful.

Bonne journée !