Samsung SMT-i6020 User Manual page 18

Call Forward
DND
USB Charging
16
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A call can be forwarded to another number when the call cannot be
received.
If this function is selected, the phone does not ring even when a call
is placed to the phone.
With a phone to which a power adaptor is connected, you can use
the USB charging function. (When PoE is connected, the USB
charging function is not available during call.)