Secondly, the current draw. Sending an SMS (a tiny burst of data) will generally draw around less than 400 mA, which is within our normal Arduino specification. If you are using your shield to make or receive actual telephone calls, the shield can draw up to two amps of current. [...]Therefore if communicating in this way you cannot run your Arduino and this shield from the USB port or the DC socket. And don't be lazy by trying it anyway.Instead, power your project with a high-capacity 5V 2 amp power supply to the 5V and GND pins on your shield.
But why couldn't use the DC socket with a 5V-12V (using the board's power regulator) adapter rated at 2 mA or more ?