on the telephone
Currently using a telephone to talk to another person. Please be quiet! Can't you see I'm on the telephone? I was on the telephone for nearly three hours trying to get through to a customer service representative.
*on the telephone
and *on the phoneFig. speaking on the telephone. (*Typically: be ~; get ~.) She's on the phone but won't be long. Please take a seat while I'm on the phone. Get on the phone and call him back immediately!