Apex domains cannot have CNAME records.
Question and response
Question
dig t-mobile.com. NS
Answer
t-mobile.com. 3 NS ns10.zayo.com.
t-mobile.com. 3 NS a13-67.akam.net.
t-mobile.com. 3 NS ns2.customer.level3.net.
t-mobile.com. 3 NS a14-65.akam.net.
t-mobile.com. 3 NS ns7.customer.level3.net.
t-mobile.com. 3 NS ns20.zayo.com.
t-mobile.com. 3 NS auth2.dns.cogentco.com.
t-mobile.com. 3 NS auth1.dns.cogentco.com.
t-mobile.com. 3 NS a6-66.akam.net.
t-mobile.com. 3 NS a7-65.akam.net.
t-mobile.com. 3 NS a8-66.akam.net.
t-mobile.com. 3 NS a1-167.akam.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0