You will have to go into programming using a 7310 or 7324 telephone. Press FEATURE **266344 (CONFIG), then reenter 266344 again. This is default password, it's possible password has been changed. You will see a menu pop up that says CONFIGURATION, if you know how to navigate through programming, press the SHOW button and you will see, TRK/LINE DATA, now press NEXT until you see LINE ACCESS, press SHOW and then SET will be prompted. At this point enter in the INTERCOM number of the set you want to change programming for. Then you will be prompted, LINE ASSIGNMENT,
press SHOW again then press the soft key below display that says "ADD", then press 008. This will add the access to the line through that set. If you want to add line to other sets you will have to follow these same steps to change each one.
You could also use the copy feature to program these changes to other sets.