Electronic – Interfacing SIM900A GSM Modem with TM4C123G TI microcontroller

Proprietary phones will not allow you to customize at all. But if all you need is to put in a SIM and log on, then I recommend to use an actual phone without even disassembling it.

Before you start reverse engineering a proprietary system, you are probably better off spending the extra money.

Shelling out over $100, I have had good results with the Arduino GSM Shield.

I have a very similar board that is on the photo you posted, but with SIM900A instead of SIM900.

You can't connect the RS232 Serial Port directly to Arduino because of mismatch in voltage. The voltage on these pins is higher than Arduino's 5V. You would need a TTL to RS232 converter to hook up Arduino to those pins.

This board has also another interface to communicate. Those 2 jumpers can be removed and then you have access to 2.8V serial pins. Looking at your photo - there are 6 of them, the bottom 2 are GND, and the top 2 are RX (the one on the left) and TX (the one on the right). The bad news is you also can't directly connect Arduino to these pins. You would need a logic level shifter to shift 5V to 2.8V. I've however used a bluetooth-to-serial module from ebay that outputs 3.3V and risked connecting it to these pins. It works smoothly.