SimCom SIM808 Hardware Design page 66

Smart Machine Smart Decision
GSM cellular terminals or mobiles operate over radio frequency signals and cellular networks and
cannot be guaranteed to connect in all conditions, for example no mobile fee or a invalid SIM card.
While you are in this condition and need emergent help, please remember using emergency calls.
In order to make or receive calls, the cellular terminal or mobile must be switched on and in a
service area with adequate cellular signal strength.
Some networks do not allow for emergency call if certain network services or phone features are in
use (e.g. lock functions, fixed dialing etc.). You may have to deactivate those features before you
can make an emergency call.
Also, some networks require that a valid SIM card be properly inserted in the cellular terminal or
mobile.
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SIM808_Hardware Design_V1.00 2014.03.27