The icon provides you with a number pad that you can use to enter numbers directly, similar to a conventional phone. Possible contacts are already suggested during input. You can also copy numbers as well as DTMF characters from other applications and paste them into the input field above the keypad.

In the field My number: you can find your number that is used for this call and is singalled to the recipient. See also To configure the number you want to signal by default.