If you are on an active call, you can do the following to switch the caller from the Aastra 6757i base station or 57i handset to the other device.

If caller is on base station and you want to switch to handset:

1. Press Hold button on base station
2. Then on your handset, look for the 'Line' indicator on the LCD screen (Ex. Line 1)
3. On your handset, press the arrow key under the appropriate Line indicator and you will be connected with the caller

If caller is on handset and you want to switch to base station:

1. Press the 'F' key on handset
2. Scroll down to the next available Line (Ex. Line 2) and press Select.  This puts the caller on hold and you will hear dial tone
3. Press the 'End Call' button on your handset
4. Then on your base station, look for the Line with flashing light and press that line button (Ex. Line 1).  You will be connected with the caller