sip.nitronet.pl has no CNAME records.
Question and response
Question
dig sip.nitronet.pl. NS
Answer
The response has no answer records.
Authority
nitronet.pl. 3600 SOA dns.nitronet.pl. hostmaster.nitronet.pl. 2025022005 3600 600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0