With a call on hold (fast-flashing appearance button) and another call connected (red call appearance button), you can switch between calls by pressing the appearance button of the held call. The current call is automatically put on hold and the previously held call is connected.

When you place a call on hold in order to make a transfer, the call is considered 'on hold pending transfer' and is indicated by fast-flashing red and green appearance buttons.  On phones with only a single appearance button (or icon), the phone displays "On-Hold-Xfer" before the call information on the call's application line.