Rasbperry Pi 3 with GSM module
Modem
It's a generic chinese modem with SIM800C GSM chip, has microphone and audio input.
Connection
Serial port
The modem uses a serial port to communicates with you, by default the serial port is redirected to a terminal port, to change it you need to follow this steps in Raspbian 8.0.
Launch raspi-config:
$ sudo raspi-config
Select 9. Advanced Options, next A7 Serial, choose no to question “Would you like a login shell to be accessible over serial?”. Select Finish button.
We need also to enable UART, follow this steps, edit /boot/config.txt and add or change the next lines:
enable_uart=1 dtoverlay=pi3-miniuart-bt
You can follow the same steps with RPi and RPi2 but omitting the “dtoverlay=pi3-miniuart-bt” line.
Restart.
minicom and serial port
Now you can start to test your modem:
sudo minicom -b 115200 -o -D /dev/ttyAMA0