mx1.oncallcentre.com has no CNAME records.
Question and response
Question
dig mx1.oncallcentre.com. NS
Answer
The response has no answer records.
Authority
oncallcentre.com. 3242 SOA ns65.domaincontrol.com. dns.jomax.net. 2025082601 28800 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0