Wired headset circuit
Electret microphones need a biasing voltage to operate: The 2.7 Volts you measured.

If there is a capacitor in series with the microphone, this would block DC, thus there would be no biasing voltage.

If the preamplifier / buffer inside the phone body needs the bias voltage removed, a capacitor would be placed in that part of the circuit, past the bias voltage provision stage, i.e. on the PCB inside the mobile phone handset.