Apex domains cannot have CNAME records.
Question and response
Question
dig sip.is. NS
Answer
sip.is. 5 NS ns03.ns.sip.is.
sip.is. 5 NS ns02.ns.sip.is.
sip.is. 5 NS ns01.ns.sip.is.
Authority
The response has no authority records.
Additional
ns01.ns.sip.is. 5 A 178.19.48.11
ns02.ns.sip.is. 5 A 178.19.48.12
ns03.ns.sip.is. 5 A 188.166.99.137
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0