Some phones have a Link button (on screen) and a soft-key button (the key below the display panel that performs special functions). If the soft-key button has been pressed, the Link button will not work. Refer to your phone manual to find the soft key button.
Release the soft-key button and you should be able to use the Link button to answer your incoming call. Some phones allow you to use the Flash button or the receiver to take the second call.