Nokia 6126 / 6131 / 6133

I think it is hidden "under" the bottom row of keys. I came to this conclusion by blowing on various parts of the phone while listening through a different phone. The wind noise was most pronounced when blowing at the bottom row of buttons. It seems to work well enough when the phone is up close to my face, but sounds like I'm talking through a tunnel when I hold the phone away from my cheek.