mx1.isvoip.net has no CNAME records.
Question and response
Question
dig mx1.isvoip.net. NS
Answer
The response has no answer records.
Authority
isvoip.net. 237 SOA ns1.is.co.za. support.is.co.za. 2022030702 28800 7200 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0