If you are looking to add a external bell style ringer to your DK16e you would need an analog circuit. You could put a KSTU in the MAIN cabinet or an RSTU in the Expansion. You will not need a receiver card (K4RCU) as it is only used for touch tone dialing.

However, you could also assign ringing to the built in page port. You would then get ringing over your paging system. The system would need to be in night mode for this.

Lastly you could hook up a PI-DKT devise to an open DKT port, and again hook it up to your paging system. You would assign ringing as you would a normal phone.