Apex domains cannot have CNAME records.
Question and response
Question
dig telefonica.com.sv. NS
Answer
telefonica.com.sv. 5 NS nssv2.telefonica-ca.net.
telefonica.com.sv. 5 NS nssv.telefonica-ca.net.
telefonica.com.sv. 5 NS ns1.movistar-sv.net.
telefonica.com.sv. 5 NS ns.movistar-sv.net.
telefonica.com.sv. 5 NS nssv1.telefonica-ca.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0