I bought my old mobile phone around the year 2000. At that point of time, a mobile phone that can be flipped open was stylish. It has a small display on the front which shows the time and when it is flipped open there is a larger display and a keypad (photos below).
The most important functions of a phone are to enable the user to call other people, answer to calls, and in some cases also to refuse a call without answering it. This last case occurs if the situation is not convenient to answer the call. For example, one is taking part in a meeting or a lecture. To call someone the phone needed to be open for obvious reasons. It is possible to answer calls with an open as well as with a closed phone. However, users need to open the phone to refuse a call. Consequently, this setting had a potential for trouble if I needed to end a call quickly.
The design might lead to another unwanted situation through the volume control buttons on the side of the phone. It was possible to activate those volume control buttons while putting the phone in a pocket. Therewith a former vibration notification could turn into a loud ringtone. My current (smart) phone similarly has the volume control on its side. However, users can only activate the volume control when the phone is not in standby which avoids an unwanted interaction.