SIM900A showing Starting modem test...ERROR, no modem answer. Checking IMEI...Modem's IMEI: 0
Note:

Detailed Solution can be found Here



Excerpt:

My Pins connections were wrong, it should not be: 0 and 1 it should be 2 &amp; 3

Also SoftwareSerial.h is not working with SIM900A

Pins:

All connections should be like this:

SIM900A - Arduino
=================
VCC     - VDC (+5V)
GND     - GND
5VR     - Connect to Arduino TX (Pin 3)
5VT     - Connect to Arduino RX (Pin 2)


Details Here