のちほどこちらからお電話___。
Q. Which humble form completes 'we will call you later'?
Translation
We will call you back later.
Explanation
いたす is the humble form of する, used for one's own (or one's company's) actions: お電話いたします. なさる/される/くださる are honorific and would wrongly elevate the speaker's own action.