What Voltage/Current Levels Can I Send Into the Microphone Input on a Mobile Phone?
Microphone inputs on phones are designed to be compatible with electret microphones which produce very small ac voltages.  A signal at about 5mV peak to peak should work.  The phone may be looking for a DC load to detect the presence of a mic - so have a 1k dc impedance to gnd. Be careful, iPhones and Android phones have different pin outs on the headphone jack.   
RS232 signals are not dc balanced and therefore cannot be captured properly by an audio input.  That is why the serial data must be FSK modulated (or similar algorithm) or Manchester encoded.
Some (or most?) phones have a ~100Hz high pass filter on the input which not only blocks dc but also causes phase distortion so that an input square wave will not look much like a square wave after capture.