Nokia 305 - Manual (Page 7)

If you only have one SIM, put the card in the internal SIM holder.
The SIM and its contacts can be easily damaged by scratches or bending, so be careful
when handling, inserting, or removing the card.
1 Put your fingernail in the gap on the right edge of the phone, then carefully lift
and remove the back cover (1).
2 If the battery is in the phone, lift it out (2).
3 Slide the SIM under the metal tab (3). Make sure the contact area of the card is
face down and the tab holds the SIM in place (4).
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