be on the phone
To be talking on the telephone. The boss is on the phone right now, but I can tell her you stopped by.
be on the ˈtelephone/ˈphone
1 be using the telephone: Mr Perkins is on the telephone but he’ll be with you in a moment.You’re wanted (= somebody wants to speak to you) on the telephone.
2 (British English) have a telephone in your home or place of work: They live on a small island and are not on the phone.