Protecting a circuit module in a communication device

A method of protecting a SIM card in a mobile phone from excess voltage comprises the steps of detecting whether a SIM card is present in the phone, measuring a supply voltage to the phone and comparing it to a predetermined threshold voltage. When the supply voltage is below the threshold, normal functionality is supplied to the phone. When the supply voltage is above the threshold a connection to the SIM card is powered down to limit functionality of the phone. Voltage comparison can be periodically checked to see if the SIM card is still present. On initial power up the phone checks for the presence of batteries and if not present the SIM (if present) is powered down. A voltage above the threshold indicates that an external charger is present.