Apex domains cannot have CNAME records.
Question and response
Question
dig telmex.com. NS
Answer
telmex.com. 5 NS ns10.telmex.com.
telmex.com. 5 NS ns9.telmex.com.
telmex.com. 5 NS ns8.telmex.com.
telmex.com. 5 NS ns11.telmex.com.
Authority
The response has no authority records.
Additional
ns8.telmex.com. 5 A 200.57.141.87
ns9.telmex.com. 5 A 200.57.141.88
ns10.telmex.com. 5 A 187.191.73.98
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0