The telephone is the electrical system of communication that you use to talk directly to someone else in a different place. You use the telephone by dialling a number on a piece of equipment and speaking into it.

A telephone is the piece of equipment that you use when you talk to someone by telephone.

If you telephone someone, you dial their telephone number and speak to them by telephone.

If you are on the telephone, you are speaking to someone by telephone.

To be on the telephone means to have a telephone in your house or office which is connected to the rest of the telephone system.