mainback.pbxhosting.com.mx has no CNAME records.
Question and response
Question
dig mainback.pbxhosting.com.mx. NS
Answer
The response has no answer records.
Authority
pbxhosting.com.mx. 9930 SOA dns1.pbxhosting.com.mx. ti.inbtel.com. 2026022105 3600 1800 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0