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+CREG: 1,1 - the second 1 means "Registered, home network"
you'll find the details in the "SIM900 AT Command Manual" ... look for: AT+CREG Network Registration
i still suspect a power supply issue. could you check the voltage drop at the power supply with an oscilloscope
and see what happens when the module turns off after "call ready"? i'm using 4.0V.
just another idea: could you use a Lithium-Ion/Polymer battery instead of the power supply...
you don't need R12, there is already a 15k pull-up resistor in the module.
i'm using a similar interface between sim and pic, but without C5,C6,C7.
could you try removing them and see if the problem is still there?
have you checked the voltage on the STATUS pin? does it become LOW with the sim card connected?
how about the POWERKEY pin? btw, after network registration the status led should blink 64/3000ms (NETLIGHT pin).
You may have some problems with the power supply. Make sure that it can provide 2 Amps necessary for transmission bursts (network registration, gprs transmit, etc). See chapter "3.3 Power Supply" in the Hardware Design documentation. A quick and dirty fix is to use a very large capacitor...
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