What makes a cellphone vibrate?

Among the many components inside the phone is a small motor.

The motor is built in such a way that it is partially off-balanced.

In other words, a mass of improper weight distribution is attached to the motor’s shaft/axis. So when the motor rotates, the irregular weight causes the phone to vibrate.

I saw a  motor when i opened the phone back cover.
Below is the image of one type of vibrating motor; there are many types.
The size is small… smaller than many currency coins.


The mass at the left end in the picture below, almost resembling a half-moon is the off-balanced weight, the cylindrical part contains the motor.
Hope this helps.