mxphx01.mdsl.net has no NS records.
NS records for this zone are defined at mdsl.net.
NS records for this zone are defined at mdsl.net.
Question and response
Question
dig mxphx01.mdsl.net. NS
Answer
The response has no answer records.
Authority
mdsl.net. 900 SOA ns-1624.awsdns-11.co.uk. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0