Apex domains cannot have CNAME records.
Question and response
Question
dig oncallcentre.com. NS
Answer
oncallcentre.com. 5 NS ns66.domaincontrol.com.
oncallcentre.com. 5 NS ns65.domaincontrol.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0