So far we have not bought any for our customers, but I found a couple phones that had a plantronics ring pickup that plugs into the CS55 base. I have seen it on their website

Here is a picture (Website linked just to show the picture)
https://www.headsets.com/headset/plantronics-rd-1-pod-ring-detector-for-shoreTel-phones/

The HL10 lifters work also. I once installed an HL10 on a DP5000 phone. It would not fit under the handset, and was not needed except for ringing, so I taped it to the back of the phone instead of on the craddle.