D-Link DPH-125MS Quick Install Manual page 8

On the Identification tab, in the User Type box, select Person from
the drop down menu if it is not already selected.
In the Name boxes, type the first and last name, and optional
nicknames or titles of the person who will use this phone. You should
specify nicknames and titles if you think callers will use when asking
for this user.
In the Extension number box, change the extension number if
desired and click OK to continue.
You have now assigned the person's name to Line 1 with the extension
number you specified. You can assign up to 3 additional users to this
phone or click Next to continue.
D-Link DPH-15MS Quick Install Guide