Look in the contacts table for a calculation that combines the first and last names, giving the last name first. If there is no such field, add one such as:
Last Name & ", " & First Name
Open Manage | Value Lists and start a new value list.
Select the "use values from a field" option and select the ID number field from Contacts as your first field, select the also use values from check box and then select your calculation field as the second field to show the full name. You'll find an option for sorting the list by the second field.
You may want to use an auto-entered calculation on a text field with the above calcualtion. If you use that option, you can specify a unique values validation. One draw back to the above value list is that contacts with duplicate full names will be omitted from the value list--only one instance of each name will appear in the list.
Please note that there are more sophisticated options that use scripting and/or relationships to produce a more user friendly way to select contacts by name, yet enter the Id number instead of the name. (And you really do need to enter the ID number in order to establish a link based ID numbers instead of names.)