Attended Transfer (Warm Transfer):
1.) Press any open line key to make a new call and the active LINE will be placed on hold automatically. (An open line can be anything that is not assigned on your phone. For the GXP2153 it will be the keys 2-4, and for the GXP1625 it will be key 2)
2.) Once the call is established, press transfer button followed by the LINE button of the held line to transfer the call.
3.) After the call is transferred, phone will display the default phone screen.