T2 features two beamforming microphones for speech. One is directed towards your mouth and the other in the opposite direction. T2 listens to both inputs, compares the delays and works out which noise is desired (your voice) and which one is undesired (the rest). This results in a crisper sound with reduced noise coming from the outside.
How to position your earbuds for the microphones to pick up your voice
The beamforming microphones in T2 are tuned to listen for speech from a certain direction and distance, approximately from your ears (where the earbuds are placed) to your mouth (where your voice is coming from). Make sure to fit the earbuds so that the pointy end (where the microphones are located) is aligned with your mouth. Wearing the earbuds incorrectly, speaking very quietly, or using one earbud at a time can affect the sound quality of your voice.
If you prefer using only one earbud, make sure to keep the other one inside the T2 charging case in the meantime.