Apex domains cannot have CNAME records.
Question and response
Question
dig voice.medallia.com. NS
Answer
voice.medallia.com. 5 NS ns-2016.awsdns-60.co.uk.
voice.medallia.com. 5 NS ns-958.awsdns-55.net.
voice.medallia.com. 5 NS ns-346.awsdns-43.com.
voice.medallia.com. 5 NS ns-1297.awsdns-34.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0