telephoning

Over the phone, which one would be correct or the most appropriate?

I'm afraid he is out today. I'll get him to call you back.
I'm afraid he is out today. I'll have him call you back.
I'm afraid he is out today. I'll ask him to call you back.

They're all technically correct. I'd say the most common response would be #2. Since he's out "today," you might want to add "tomorrow":

"I'm afraid he's out today. I'll have him call you back tomorrow."

Another possible response could be: "I'll tell him to give you a call back tomorrow."

Like usual, there are many ways to express the same idea.

Or you could say...
"I'll have him ring you back."
"I'll have him ring you up."
"I'll have him give you a ring."

Or...
"I solemly swear that you'll either get to hear from him or a pound of my flesh by tomorrow!"