When calling the "boss", the delegate's Skype for Business client will show the incoming call but the deskphone will not ring. When a different "boss" calls the same client, it will ring both the Skype for Business client and the phone. I have also added a different delegate to the "boss" having the issue and the phone rings just fine.
I have tried factory resetting the device, firmware versions (220.127.116.1173, 18.104.22.16848, and 22.214.171.12430), and removing the delegate + re-adding. I have had no luck.