Understanding voltage levels on 3.5 mm audio output
In regular line level audio, DC voltage does not serve a purpose. 

It's conceivable that some devices may use a DC offset in order to detect activity and to go into standby - or turn on - depending on whether there's a DC offset or not, without having to depend on an actual audio signal to be present at all times. But I'm not aware of any standard for this.

Audio output circuitry usually does contain DC blocking capacitors. 

(But not always; certain high end "audiophile" power amplifiers do not. With these amplifiers, any DC voltage offset on the input signal can easily blow the speakers. Yes, it's a dumb design.)

A headphone jack does not output line level audio though; it's a low(er) impedance output designed to drive tiny speakers. If the DC voltage on the output of your phone does serve any purpose, I'm guessing that it might have to do with the small remote control for receiving and ending calls, and changing the volume, often found on the cord, although there's often a separate wire for that (mini-jack with four conductors).