ns.pingtone.com has no NS records.
NS records for this zone are defined at pingtone.com.
NS records for this zone are defined at pingtone.com.
Question and response
Question
dig ns.pingtone.com. NS
Answer
The response has no answer records.
Authority
pingtone.com. 900 SOA ns.pingtone.com. dnsmaster.pingtone.com. 1734668980 10800 3600 604800 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0