Hello,

If iPhone is set to ring mode, during an incoming call, do the following to silence the ringtone and hear Caller ID Info:

1.
Tap either the Sleep/Wake button or a Volume key once.  This will only silence the ringtone.  You may still answer the call.  

2.
Tap the upper right corner of the touch-screen and VoiceOver will announce the Caller ID each time the screen is pressed.

NOTE:
To send a call to VoiceMail after having heard its Caller ID, tap either the Sleep/Wake button or a Volume key once more.

NOTE 2:
Upon receiving an incoming call you may send it immediately to VoiceMail, by tapping either the Sleep/Wake button twice.

Mark