I'll call back later
I'll call (someone or some place) on the phone again, usually after failing to reach the desired person the first time. I didn't get an answer, so I'll call back later. Everyone in the passport office was out to lunch, so I'll call back later.
I'll call back later.
a standard phrase indicating that a telephone caller will call again at a later time. Sally: Is Bill there? Mary (speaking into the telephone): Sorry, he's not here right now. Sally: I'll call back later. John (speaking into the telephone): Hello. Is Fred there? Jane: No. Can I take a message? John: No, thanks. I'll call back later.