mail-vtr2.voip.ms has no CNAME records.
Question and response
Question
dig mail-vtr2.voip.ms. NS
Answer
The response has no answer records.
Authority
voip.ms. 285 SOA chan.ns.cloudflare.com. dns.cloudflare.com. 2398526707 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0