How can I build a FBus to USB cable?
In most hardware-related ways, FBus is like RS-232 (sorry, EIA-232). The signalling levels are different, and FBus mandates 115200N81 communications with signals at 3.3V. Everything else is software (a packet-based format).

You can cobble together an FBus-to-RS-232 cable with a single MAX232 chip using the textbook implementation from the MAX232 datasheet.

For USB, an FTDI 3.3V serial-to-USB active cable (a tiny PCB with an FTDI chip is in the plug assembly) might work without any external components other than the appropriate plug for the phone you have in mind. Any other 3.3V USB to ‘TTL’ serial converter would work equally well.

Be sure to read the datasheets though! Standard RS/EIA-232 signalling levels can vary anywhere between ±3V and ±15V and your FBus device will not be that tolerant.