Apex domains cannot have CNAME records.
Question and response
Question
dig metaswitch.com. NS
Answer
metaswitch.com. 5 NS ns-1631.awsdns-11.co.uk.
metaswitch.com. 5 NS ns-1420.awsdns-49.org.
metaswitch.com. 5 NS ns-145.awsdns-18.com.
metaswitch.com. 5 NS ns-621.awsdns-13.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0