sip.datakom.ru has no CNAME records.
Question and response
Question
dig sip.datakom.ru. NS
Answer
The response has no answer records.
Authority
datakom.ru. 112 SOA ns1.luadns.net. hostmaster.luadns.com. 1772509623 1200 120 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0