GSM Shield not taking calls from other phones

Question

I am using Arduino Uno R3 with the SIM900A for a couple of weeks. The problem I'm facing is, everytime I make a call from a Nokia (or anyother) cellphone to SIM900A - I see just one RING as unsolicited result code on the Serial Monitor and I cannot hear any rings going on in Nokia. Everything is blank for about 5 seconds and then the call attempt drops.

Why is this happening? Do I need to execute some AT Command as soon as a RING happens, to show Calling Cellphone that the ME is ALERTING/RINGING ? Also, when I call once to SIM900, should I not see many "RING" instead of once?

Answer 1

Finally, I found out what was wrong. External Power Supply!

Turns out the SIM900A board was not getting enough current to function properly. When I connected the Arduino Board to 12V external power Supply (2Amps), it worked like a charm!

Try it out, it really works! :)