The power supply of SIM900 is from single voltage source of VBAT= 3.4V TO 4.5V. In some case, the ripple in a transmitting burst may cause voltage drops when current consumptions rises to typical peaks of 2A. So the power supply must be able to provide sufficient current up to 2A.
Yes, that is correct. The peaks are of brief duration but unless the supply has sufficient capacity you can have intermittent problems.