There’re several ways to silence the ringer before answering a call.
Method 1: Press the Volume Down/Up or Power button on the side of your device.
Method 2: Go to menu -> “Settings” -> “Sound & vibration”-> activate “Silent mode”.
Method 3: Drag the Status Bar down with your finger and then touch the “Sound” icon to change to “Vibrate” or “Mute”.
Note: Press the “power” key and “volume up” key at the same time to switch to vibration mode quickly.