Wiktionary
on the horn
a. (context idiomatic English) On the telephone.
Usage examples of "on the horn".
As Will gazed, he raised his great head, yellow light flashing from tawny round eyes like the eyes of an owl, and he gave a call that was like the halloo a huntsman blows on the horn to call up hounds.
I need to clear this place of the Host so that I can get my hands on the Horn.
I need to get Orc, your beloved brother, out of that trap so I can get my hands on the Horn.
I need to get Ore, your beloved brother, out of that trap so I can get my hands on the Horn.