To operate the serial port, we need to run an RS232 to TTL converter. Fortunately, there's a 3.3v output on the bottom of the phone that'll power our unit. Not useful if you want to get into the phone's serial console, since it only provides power once booted.
The voltages in regular RS232 serial would fry a TTL port quick smart, so I needed something to convert between the two. Maxim makes a chip called a MAX3232 that will do the job. You can buy one of these pre-soldered to a DB9 connector and PCB, or you can buy the raw chip and wire it up yourself. The raw chip is cheaper but again, more work. You'll need five 0.1uF capacitors and the knowledge of how to wire them up. This information is pretty readily available, but the datasheet for the MAX3232 is confusing, so here's a schematic on what this raw wiring should look like. There are plenty of prebuilt ones on eBay, but they all have female DB9s, so I had to make up a custom headshell.