RS-232C with phone connector
It's likely a 3.5mm audio jack also called a phone connector as you say.  It's common to use a cheaper connector to add (an often unused by the consumer) serial port.  Anyway if you want to use it, grab an old audio cable or a pair of headphones and cut the cable.  Or you could get yourself a 3.5mm audio plug from digikey.  Then just ohm it out to find gnd and then connect to either a scope or a serial device and try to figure out which pin is gnd and which is receive.  There's only a couple of combinations of RX, TX and GND.