ns0.bt.net has no CNAME records.
Question and response
Question
dig ns0.bt.net. NS
Answer
The response has no answer records.
Authority
bt.net. 600 SOA ns0.bt.net. hostmaster.bt.net. 2017032640 28800 7200 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0