Apex domains cannot have CNAME records.
Question and response
Question
dig gvp.telefonica.com. NS
Answer
gvp.telefonica.com. 5 NS ns2-201.azure-dns.net.
gvp.telefonica.com. 5 NS ns4-201.azure-dns.info.
gvp.telefonica.com. 5 NS ns1-201.azure-dns.com.
gvp.telefonica.com. 5 NS ns3-201.azure-dns.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0