Apex domains cannot have CNAME records.
Question and response
Question
dig main.pbxhosting.com.mx. NS
Answer
main.pbxhosting.com.mx. 5 NS dns3.pbxhosting.com.mx.
main.pbxhosting.com.mx. 5 NS dns1.pbxhosting.com.mx.
Authority
The response has no authority records.
Additional
dns1.pbxhosting.com.mx. 1 A 45.77.185.146
dns3.pbxhosting.com.mx. 1 A 172.105.19.115
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0