sipexternal.bluewirecs.com has no CNAME records.
Question and response
Question
dig sipexternal.bluewirecs.com. NS
Answer
The response has no answer records.
Authority
bluewirecs.com. 1284 SOA ns1.scc-zip.net. dnsadmin.scc-zip.net. 2012121243 28800 7200 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0