Apex domains cannot have CNAME records.
Question and response
Question
dig bt-telekom.com.tr. NS
Answer
bt-telekom.com.tr. 4 NS dnx2.vodafone.net.tr.
bt-telekom.com.tr. 4 NS dnsa1.vodafone.net.tr.
bt-telekom.com.tr. 4 NS dnsa2.vodafone.net.tr.
bt-telekom.com.tr. 4 NS dnx1.vodafone.net.tr.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0