However, this is not typically how you would say it in Italian. Here are some more natural ways to express the idea depending on the context:
* "Sono al telefono" (I am on the phone) - This is used when you are currently on a phone call.
* "Sto chiamando" (I am calling) - This is used when you are in the act of making a phone call.
* "Ho telefonato" (I called) - This is used to indicate that you made a phone call in the past.
* "Mi ha telefonato" (He/She called me) - This is used to say that someone called you.
It's important to consider the context and what you are trying to convey when translating "on telephone" into Italian.