These modems would often slot into a Mini PCI ... 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 card ...

The new Pi offered a number of compelling hardware upgrades, including an onboard PCI-Express interface. The only problem was that the PCIe interface was dedicated to the USB 3.0 controller ...

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.