Bull crap. I just took out my cell phone and started punching keys (just to prove my point) and suddenly everyone just looked over at me wondering what I was doing. I am sitting in an office with five other people and they all could hear it. Texting is not a silent activity.
Did you actually ask your mates if their notice was due to HEARING... or SEEING?
It is silent on my phone, but it is a touch screen.