I am
trying to set up forwarding of a SIP phone to an external (PSTN) phone number
using the SIP REGISTER (rather than the usual 302 Moved Temporarily response)
for a rather unconventional application. 
 
Is
there a way to register an external phone number as a contact number on the
CISCO 29xx series integrated router (IOS 15.1(4)M1)? 
 
I've
tried using the #############Tel URI in the Contact Header field of the SIP
REGISTER and received a 400 Bad Request - 'Malformed/Missing Contact
field".  (I've tried every variation of the number and get the same
response.)
 
I've
tried using the SIP URI in the Contact Header field, using the following
format:  TelephoneNumber@IP_address_of_Cisco_gateway
and receive 404 Not Found.  In the debug messages, I'll see, "contact
address matches own ip address" (but what IP address can I use for an
external phone number?).
 
Any
suggestions on the best way to set up a REGISTER for a SIP phone to have the
Contact header field containing an external (PSTN) number?