mail.choruscall.com has no CNAME records.
Question and response
Question
dig mail.choruscall.com. NS
Answer
The response has no answer records.
Authority
choruscall.com. 900 SOA cbru.br.ns.els-gms.att.net. rm-hostmaster.ems.att.com. 405 3600 600 1209600 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0