I am using Arduino Uno R3 with SIM900A for a couple of weeks (GSM/GPRS TTL UART MODEM - SIM900A (rhydoLABZ) [GSM-1965] : rhydoLABZ INDIA) . 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?
How are you powering it?
If you are just using the SIM900 using USB power from the board, this is almost your problem
Sim900 need some amps to operate properly
Try to power the board or the arduino itself using an external power source with at least 2Amps
Looks like you were ABSOLUTELY RIGHT about it @HugoPT !! I connected the Arduino board to 12V - 2 Amp external power supply and VOILA! everything is working as it should.