sip.6level.net has no CNAME records.
Question and response
Question
dig sip.6level.net. NS
Answer
The response has no answer records.
Authority
6level.net. 201 SOA ns1.6level.net. admin.6level.net. 2026022300 3600 300 2419200 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0