What is the button called that the receiver goes on to hang up the phone?

In telephony, the "thing" the receiver is placed upon after hanging up is called the "hook" because on the old wall phones it literally was a hook. If you neglect to hang up a phone, it is said to be "off hook" and this condition can be detected at the central phone switch. Likewise, a phone properly stowed and inactive is said to be "on hook". Even though we no longer stow our phone receivers on a hook, the terms "on hook" and "off hook" are still used in the telephone business.