I'd like an answer to this as well having the same phone and same issue.
Unfortunately I suspect the answer is that it's not enabled because we didn't purchase the phones from BT, despite the Nexus 5x being listed by BT as supporting wi-fi calling. http://bt.custhelp.com/app/answers/detail/a_id/60450/related/1
have you read this large post https://community.bt.com/t5/BT-Mobile/Wifi-Calling/td-p/1638964
As pointed out, only a few handsets purchased directly from BT Mobile will officially support Wifi &4G calling due to the firmware on the device. Currently you need to reflash your (those not officially supported) handsets with a generic or EVR Firmware (EE's firmware) and the correct APN to make it work on other handsets.
Current Officially supported handsets are A8 (2018) S9/S9+ and iPhones 5S and above. Other hardware will offer these services with the correct firmware. If you change the firmware, BT can no offer any support to help get wifi calling working. If you are confident in doing so, use the web to look up for generic firmware and ensure your device is backed up prior. Or find a store that will update it for small fee..