Apex domains cannot have CNAME records.
Question and response
Question
dig att.net.co. NS
Answer
att.net.co. 4 NS ns3.telmexla.net.co.
att.net.co. 4 NS ns2.telmexla.net.co.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0