Acer Liquid Z6 - Manual (Page 9)

Setting up - 9
5. Replace the cover by aligning the tabs on the cover into the slots on
the phone. Gently press the cover until it snaps in place.
SIM card lock
Your phone may come with a SIM card lock, i.e., you will only be able
to use the SIM card provided by your network operator.
To cancel the SIM lock, contact your network provider.