vpn.sip.com.tr has no NS records.
NS records for this zone are defined at sip.com.tr.
NS records for this zone are defined at sip.com.tr.
Question and response
Question
dig vpn.sip.com.tr. NS
Answer
The response has no answer records.
Authority
sip.com.tr. 1800 SOA clayton.ns.cloudflare.com. dns.cloudflare.com. 2396970007 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0