Introduction to induction wire neckloopTP21 comes with new jack,3.5mm 4pins the key feature of TP21 is built in Microphone.Including Answer Button and AV Switcher.2 colors available,beige and black.AV Switcher: control the switch for different mobileAnswer button: short press it to receive the call. For some mobile phones,to press twice or three times of the button can realize the controlling the up or down of playing music.Plus 3.5mm jack into headset socket in mobile phone,then confim the neckloop is for "Audio Output / Input ",if mobile phone remind you to choose the headset type. Package Includes:Basic neckloop is packed in a plastic bag. Each set include below items.If anything missed please contact with seller.1 x TP21 basic neckloop with microphone clip and microphone sponge cover.1 x 218 earpiece How to use induction neckloop earpiece kit?Firstly install button battery into earpiece and place the micro earpiece in your ear canal. Then wear induction loop on your neck and connect loop to mobile phone or mp3 player. Default jack for neckloop is 2.5mm. If your mobile phone / mp3 needs other models of jacks, please use jack adaptor on neckloop. Then put mobile phone / mp3 player and induction neckloop under the cloths. Turn on mp3 player or dail a phone number in mobile phone, after connected, you can hear voices in invisible earpiece.Mechanical Specifications of Induction Wire NeckloopLoop Length: Dimension: 820mmHandsfree Length: 100mmMax Microphone Length: 20mmWeight : 129gramPack Bag Size: 155x103x23mmNote: An inductive hearing loop can not works along but must works with wireless invisible earpieces and audio signal source. Please browse different wireless invsible earpieces by click the menus above inductive loop menu. Signal sources incluidng Mobile Phone, MP3 Player, Walkie-Talkie etc.