This is a private interconnect between Sprint and MCI, and the serial
subnet comes out of address space assigned to MCI (so the inverse
mapping is handled by MCI as well). We didn't really know what map
the inverse address to in the PTR record, but we wanted it to have
*some* mapping, so we just picked an arbitrary hostname.
We'd be happy to change this mapping to some other hostname (or CNAME
the inverse address), if the Sprint folks would prefer something
different 
The other end of the link has a more-descriptive DNS entry:
roy@ns% host -t ptr 89.77.157.206.in-addr.arpa
89.77.157.206.in-addr.arpa PTR core4-hssi5-0.WillowSprings.mci.net
                                - roy -