Newbie: How to add PTR records

[ram]

My company owns many class C IP ranges acquired from APNIC 
but only 2 have been in use. We run our own DNS hosting with PTR's for
these class C zones  working fine

Now we want to use one more class C IP 

I have created the PTR records on my nameserver 
dig -x $IP works fine on the DNS server 

 but I have to get the $REVERSE_IP_CLASS.in-addr.arpa zone NS record
pointed to my name server. So that other servers can start looking up
the new PTR records

The person who had done this for my company has long left our
organization

Can someone help me  , whom should I contact ? APNIC ? My Domain
registrar  ?  Excuse me if this is a dumb question






Thanks
Ram