backend.bbtel.com has no CNAME records.
Question and response
Question
dig backend.bbtel.com. NS
Answer
The response has no answer records.
Authority
bbtel.com. 2155 SOA ns1.bbtel.com. adminblue.bbtel.com. 520800125 1200 180 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0