What is the impedance of audio output of cell phones
In audio applications it is the goal to control the voltage over the speaker or the headphone as exactly as possible. Because the impedance (alternating current resistance) of speakers and headphones is frequency dependant, it's not advisable to achieve impedance (and power) matching.

While the output impedance of phones, MP3 players etc. is usually in the range of several ohms, the impedance of headphones goes usualy from 20 to several 100 ohms.

The headphone manufacturer has (among others) one problem, high efficient speakers are expensive to manufacture. A headphone with low impedance could be produced cheaper, but the frequency response will also be worse. A headphone with a high impedance will eventually not produce enough volume.

Generally speaking headphones with a higher impedance could have a better frequency response, but as usual ther are other points ...