Apex domains cannot have CNAME records.
Question and response
Question
dig voip.net.do. NS
Answer
voip.net.do. 5 NS pns43.cloudns.net.
voip.net.do. 5 NS pns41.cloudns.net.
voip.net.do. 5 NS pns42.cloudns.net.
voip.net.do. 5 NS pns44.cloudns.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0