AASTRA 4225 Vision for MX-ONE (TSE) User Guide EN - Manual (Page 88)

Other Useful features
Dialog 4223 Professional/Dialog 4225 Vision 88
Additional Directory Number
You can be assigned (preprogrammed by your system administrator)
one or more Additional Directory Numbers (lines) on free function
keys.
To answer, make calls and use features on the additional directory lines,
use the same procedure as for the Triple Access Line, unless you have
other instructions. However, you have to press the additional Line key
after lifting the handset.
Example:
To make a call on an additional directory line:
u
Lift the handset.
Additional line
Ô
Press.
Note: The function key is preprogrammed by your system
administrator.
z
Dial the extension number.