Apex domains cannot have CNAME records.
Question and response
Question
dig dns4.pbxhosting.com.mx. NS
Answer
dns4.pbxhosting.com.mx. 5 NS dns3.pbxhosting.com.mx.
dns4.pbxhosting.com.mx. 5 NS dns4.pbxhosting.com.mx.
Authority
The response has no authority records.
Additional
dns3.pbxhosting.com.mx. 5 A 172.105.19.115
dns4.pbxhosting.com.mx. 5 A 45.79.182.42
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0