Sandy,
ns5 and t2 seem to have worked.
I think it wants the PTR record to resolve back to the FQDN of the name server. I ran
dig a <NS>.zoneedit.com +short | nslookup
on each NS in your list, and the following are probably bad for the .is registry:
ns11.zoneedit.com -> b.dnsvr.com
dns3.zoneedit.com-> multiple records (ns4 dns4) -- not 3
ns7.zoneedit.com -> pdns2.zoneedit.com
ns12.zoneedit.com -> pdns3.zoneedit.com
ns16.zoneedit.com -> pdns1.zoneedit.com
These might be okay, not sure how it feels about mutiple PTR records
dns1.zoneedit.com -> multiple records (ns1 dns1)
dns4.zoneedit.com -> multiple records (ns4 dns4)
dns2.zoneedit.com -> multiple records(ns2 dns2)