In some place, the store or shop provide self-service. Each customer gets a number and find a place to wait. When it is his turn, the server will call his number, but don't know where he is. Maybe the customer cannot hear the call while he is doing something else.

In such occasion, there is a 0~9 keyboard transmitter, press 3 number of them, you can get from 001 to 999 different numbers. Like press 158, you can call the number 158 customer. The customer gets a mini receiver which labeled 158. When there is a call. The receiver will vibrate, he will go to service center and give back the number receiver.