key tones during calls - only audience hears

[BostonBiz]

I have an HTC Thunderbolt and a couple weeks ago one of my coworkers told me he always hears keys pressed during our calls - for the last year! I had no idea because I don't hear them, but people on the other end of the call do. Verizon couldn't figure it out, so they replaced the device. Yesterday on a conference call I was told it did it again -- now I'm wondering if it is something with the phone lines in the New England area?

Factors I've ruled out:

• my cheek hitting the screen - I'm on a headset sitting at a desk using my phone - not even holding it.
• my headset - I've had several headsets, too, and it still does it.
• an app - the original device was rebooted and now I have a "like-new" device with only a couple apps (facebook, instagram) and it still does it

Anybody else have callers report to you that it sounds like you're pressing keys but you don't hear it??