asterisk users - Oct 2004 - Specifying different SIP packet destination from hostname in request line?

I'm stumped.

I'm trying to configure my Asterisk setup to use a new embryonic UK ITSP
(who wants to remain nameless for the moment ...).

They supply their own UA, which works fine.

But Asterisk is proving to be a problem. Ethereal traces show that their UA
is sending SIP packets direct to an IP address aaa.bbb.ccc.ddd, but that the
address in the REGISTER and INVITE methods is specifying a hostname, eg:
   REGISTER sip:davidgu@ukitsp.net SIP/2.0
and
   INVITE sip:0123456789@ukitsp.net SIP/2.0

Unless I've missed something, there doesn't seem to be a way to tell
Asterisk to use a different destination for the SIP packet to that specified
in the method.

Is there a way to accomplish this?

Thanks

--
David Gurr
Congruity Ltd.
Hemel Hempstead
UK