Behind the NAT registration

 Hi All,
   Assume a scenario where SIP UA is running behind a NAT. At the time of
registration, what should be the address put in "Contact" header i.e.
local/public IP?
 If UA populates its local IP in "Contact" header, then that can't be used
to terminate call.
In general, its UA who determine its NAT bindings and poupulate in "Contact"
header or registrar determine it from source of IP packet and send back in
"Contact" header in 200 OK?

What is the best way to enable call termination on this UA behind NAT
i.erport,stun,etc?

Thanks a lot,
/raj

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