Apex domains cannot have CNAME records.
Question and response
Question
dig voip.fccn.pt. NS
Answer
voip.fccn.pt. 1 NS ns02.fccn.pt.
voip.fccn.pt. 1 NS ns01.fccn.pt.
Authority
The response has no authority records.
Additional
ns01.fccn.pt. 1 A 193.136.192.40
ns02.fccn.pt. 1 A 193.136.2.228
ns01.fccn.pt. 1 AAAA 2001:690:a00:4001:0:0:0:200
ns02.fccn.pt. 1 AAAA 2001:690:a80:4001:0:0:0:200
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0