You have been observing, that the touch screen mobile phone do not work with any thing else except using hand or special pen or pointers that come with the phone.
Soft touch Mobile phone of today are made with capacitive touchscreen, therefore when the tip of your finger get close to it or touch the screen it will create an electrical disturbances on the screen, the disturbance is then converted by the screen to a signal which you phone now responds just as human being responding to stimulus.
Now actually i am explaining to you the capacitive touchscreen, but they is another type of touchscreen called resistive Touchscreen. This is the type of touch screen usually used by electronic signing system like the one you sign when you register for you BVN, thats for Nigerian and your ATM screen.
The resistive touchscreen actually work by 2 electrical conductive layer bend to touch each other and it will create a signal which makes it work or react, before now phones where made using this pattern, but capacitive touch screen has replace that on modern phones. Note that the Resistive touchscreen do not obey finger touch but capacitive touchscreen obey finger touch.
Reason is that your finger is a skin and human skin conduct electricity so conductive touch screen only work with materials which conduct electricity, like fingers, -ve side of a dry cell Battery, metallic panel of a Sim card, etc.