While RS-232 has higher voltage, on the Teensy 4.0/4.1/micromod (and on the legacy Teensy 3.0,3.6, and LC), the voltage on the TX and RX pins must be 3.3 volts. If you use higher voltage, you risk burning out the Teensy. Only on the legacy Teensy 3.1, 3.2, and 3.5 can you feed it 5 volts. Even on the legacy Teensies that can accept 5 volts, they cannot accept 12 volts.
Thus if you want to hook up a serial device that uses 5 volts, you need to drop the voltage on the incoming data (RX), and possibly boost the voltage on the output (TX).