I have been doing NEC DS and DSX about five years now.

Assuming that you are only using one ring mode, do not DIT your lines. Set them to key ring with overflow to 700. Go to the station programing and set each line to ring on the phones you want it to ring on. You can also physically go to the phone(s) you want to ring and turn ringing on or off (this is a user programmable feature). You will have to pay with the timers to get the number of rings you want.

NEC has online training on their web site, "necdsxsupport.com"