OK, I got this module running. :D
I connected the module with a USB-Serial-Converter (TTL-Level!) to my PC.
Tx ---> SIMR
Rx ---> SIMT
GND ---> GND
I connected the Pins VCC4,VCC5 and VCC_MCU together to my power supply with a Voltage of 4.1V. And GND of the module to the GND of my power supply.
Then I powered up the module and got a connection. If you send AT (CR+LF) yout get the answer "AT OK". It doesn't matter which baud rate you have selected because the module has auto baudrate detection.
My Module was locked and I had to flash the SIM900-Firmware on my SIM900A. The original firmware version on my module was: 1137B03SIM900A64_ST_ENHANCE
I used the appliction "SIM900 Series download Tools Develop 1.9" and flashed the firmware version 1137B01SIM900M64_ST_ENHANCE
Here is a good article for flashing new firmware: http://amichalec.net/2014/08/sim900a-fixed-for-europe/
Here you can find different firmwares for the SIM900: http://dostmuhammad.com/blog/sim900-firmware-update-tutorials-appnotes/
I'm not sure if there is a voltage regulator on this module!? How do you now that the module can handle a supply voltage of 5V on VCC4 and VCC5 ? (The range of the power supply for the SIM900 is 3.2V to 4.8V as specified in the SIM900 data sheet) I have no manual/datasheet for this module as mentioned earlier...