Quectel M20 Hardware Design page 14

M20 Hardware Design
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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