a-ns.bt.com has no CNAME records.
Question and response
Question
dig a-ns.bt.com. NS
Answer
The response has no answer records.
Authority
bt.com. 336 SOA EDDNS0.BT.COM. zzdnsr.bt.com. 2006031109 28800 7200 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0