SIM900A SMS/GPRS Module

VCC : GSM module can supply terminal input voltage range of 3.7 to 4.2  Current 2A

GND: power supply negative

TX: GSM transmitter module, the corresponding microcontroller or computer RX  receiving end

RX : GSM receiving end of the module, the corresponding microcontroller or computer TX  transmitter

Headphone : sound output, you can solve the sound problem ceramic capacitors in series

Microphones : Audio input Series ceramic capacitors can solve quality problems

Reset: module boot pin to two seconds after the release of which means that, you can always grounded, so that the power is automatically turned on