Now No need to replace the back cover. fits between the battery and the back! Switch phone off and on to switch cards. Dual SIM Card Holder for mobile phone is greatly helpful for user who needs to use his (or her) mobile phone in different city, province or country for saving long-distance call cost. It can be used very simply. After inserting 2 SIM cards into your mobile phone, just press the ON/OFF key (phone power switch) to select the SIM card you prefer to use. Set call divert on mobile if you want to use two SIM card synchronously on one mobile phone and divert the second SIM card calls to the first card when the first card is working . This mobile phone accessory is compatible with Nokia 3310,3315,5510,6210,3350,3330, , 7789, 7689 and Sony Ericsson T189 mobile phones.'