Apex domains cannot have CNAME records.
Question and response
Question
dig jazztel.com. NS
Answer
jazztel.com. 5 NS dns2.jazztel.es.
jazztel.com. 5 NS dns.jazztel.es.
Authority
The response has no authority records.
Additional
dns.jazztel.es. 5 A 212.106.192.250
dns2.jazztel.es. 5 A 87.216.7.2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0