Apex domains cannot have CNAME records.
Question and response
Question
dig telefonica-data.com. NS
Answer
telefonica-data.com. 4 NS esifw1.tsai.es.
telefonica-data.com. 4 NS esifw2.tsai.es.
telefonica-data.com. 4 NS ns2.ustdata.net.
telefonica-data.com. 4 NS rh.xgn.telefonica-data.com.
telefonica-data.com. 4 NS isint30.telefonica-data.com.
Authority
The response has no authority records.
Additional
esifw1.tsai.es. 4 A 213.0.43.37
esifw2.tsai.es. 4 A 213.4.194.5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0