Headphone port of Pinephone can switched to serial port with dip-switch, I know there is cable for USB-port that make it look to PC as serial port.

But my question what hardware needed to make headphone-port more like standard RS232C serial port (accept +/- 15V input) or midi i/o ports (31,250 baud 5V) or RS-422 (+/- 6V), now port is 3.3V. I think software side is ready for that.

Is this what you were looking for? It describes the serial port configuration, including the pinout and signal levels.

It sounds like you want a signal level shifter. It is pretty simple to make if you have an electronics background, or you can get some off-the-shelf product, such as this (note I don't know anything about that product, I only just found it by typing "3.3V to RS232" in DDG).

I think I tried same query with DDG some days ago and I didn't get that, it looks good "Maximum RS232 voltage range of the IC can receive is –25V to +25V" so I don't brake it easily...it seems that it only need power of 3.3V, so it should look almost ready solution. Although RS232C port should be male not female. Anyway thanks.