Hello Miriam,
open the Template of the Phone.
You will find "Label = %%extension_number%%"
Try and set it to "Label = %%extension_first_name%% %%extension_last_name%% - %%extension_number%%"
There is Limited Space to show Informations next to the Line Key. May there is not enough for both given and surname but you can play with this as you wish and check the result.
The Templates you find on ur hard drive in an 2008 or Windows 7 at C:\ProgramData\3CX\Data\Http\Templates\phones
Close all mng console of the time you change the template. after reopen the mng and re-provision the phone from the phone tab inside the mng console