I don't notice them when my phone is in my pants pocket, and I don't have "phantom vibrations" when the phone's not in the belt pouch. I can't detect a hint of them when I'm holding the phone in my hand.
It MAY just be an awareness of pressure, or the antenna poking into my hide. There might be a blood vessel right about there, and having something sitting on top of it lets me feel the pulse.
It's very strange.
Hmmm. I just swapped my keys and my cell phone, and now I'm "receiving" the same vibration from my KEYCHAIN. That suggests that the "blood vessel" explanation is correct.