The phone is an electrical system that you use to talk to someone else in another place, by dialling a number on a piece of equipment and speaking into it.

The phone is the piece of equipment that you use when you dial someone's phone number and talk to them.

If you say that someone picks up or puts down the phone, you mean that they lift or replace the receiver.

When you phone someone, you dial their phone number and speak to them by phone.

If you say that someone is on the phone, you mean that they are speaking to someone else by phone.

If you are on the phone, you have a phone in your home or place of work, so that you can be contacted by phone.