Depends on whether you have the Electra Professional Level I or Electra Professional Level II. The Pro I does not support caller ID. The Pro II could be outfitted with a caller ID card. Google Electra Pro I and Electra Pro II and you will see the differences in what the cabinets look like to see what you have. The 16DC phone will work on either system but will only shoe CID if the proper trunk card is installed (provided it's a Pro II system).

This might be a good time to get the customer to upgrade. Even if it is to a newer refurbished system.