Apex domains cannot have CNAME records.
Question and response
Question
dig linphone.org. NS
Answer
linphone.org. 5 NS ns-229-b.gandi.net.
linphone.org. 5 NS ns-183-c.gandi.net.
linphone.org. 5 NS ns-34-a.gandi.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0