Apex domains cannot have CNAME records.
Question and response
Question
dig pingtone.net. NS
Answer
pingtone.net. 5 NS ns1.megapath.com.
pingtone.net. 5 NS ns3.megapath.com.
pingtone.net. 5 NS ns2.megapath.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0