These modems ... USB. No problem, just get a low-dropout 3.3V regulator and run that off the USB port. Then, it’s a simple enough matter of figuring out which pins are used to talk to the SIM ...

Typically available as an external USB module, for desktop computers, the modem may also be available on a PCI or PCI Express (PCIe) card that plugs into an empty slot on the motherboard.