sip1.ix1.inferno.tel has no NS records.
NS records for this zone are defined at inferno.tel.
NS records for this zone are defined at inferno.tel.
Question and response
Question
dig sip1.ix1.inferno.tel. NS
Answer
The response has no answer records.
Authority
inferno.tel. 1800 SOA woz.ns.cloudflare.com. dns.cloudflare.com. 2399075269 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0