[Asterisk-Users] Cisco 7960 won't register as SIP device

[List Receiver]

I've got a problem with my Cisco 7960 phone and SIP registration.  The
phones are at a branch office site, behind a NAT device.  No port
forwarding or redirection is set up on the NAT device.

The phones can connect to * and authenticate just fine.  They can call
any destination in the dialplan.  They appear to function fine, with the
exception of never registering as a SIP device.  A run of sip show peers
shows them as a status of offline and unknown.  Obviously, no one can
call to the phones...they can only initiate calls from the phones.

I'm sure I'm missing something simple, but searches at various *
information stores hasn't enlightened me.  Any ideas?

Thanks,
Ty