I'd add that this feature is called Automatic Line Selection. Whenever the phone rings and the handset is picked up, it picks the line that is ringing. If there are multiple calls ringing at the same time, it picks the line that has been ringing the longest.
The Automatic Line Selection is a built-in feature. It is always on and it cannot be turned off, so it does not appear anywhere in programming.
However, as curlycord pointed out, lines can be set to appear and ring, appear only or ring only. When a line appears on a phone but does not ring, picking up the handset never picks that line. The only way to pick the line is to press the line button.