Ringing bells

Shaman ritual bells (무당방울)mudangbangul, the Korean term for shaman ritual bells. can be found in various shapes. Some are gold-looking, some have more bells than others and some have colorful accessories. Regardless of the way they look, the ritual bells all have the same purpose: assisting the shaman in contacting spirits. The precise function of these bells in the process of a shaman getting possessed by a spirit is not exactly clear. As the spirits that are possessing these shamans are worshiped, some shamans may say the ringing of the bells is a way of showing courtesy. Other shamans have said the sound simply makes it clear to the spirits that they wish to be possessed by them. Generally, it is a tool specifically meant for calling upon the spirit. But what is it that makes these bells capable of doing this? Surely you would not be able to just buy a pair yourself… or would you?