Mosfet is easy. Chose one with low Vgsth and as low as possible Rdson. Current more than 4A for modem only.
I usually use AO3401. To be safe I put 2 parallel. Also it is good idea to PIC have control over them. SIM900D had sometimes issue where it would stop responding, and continue to work for months after power off on.
So from then I always make modem PSU controled by PIC, and add discharge resistor. Sometimes I also make resistor PIC controlled, if it battery powered device.