Asterisk + Nat + SPA2000 Registration Problems


I have a small office setup with a user connecting via internet from his home using a spa2000.

The asterisk server is behind Nat at the office.
Have a IAX provider, which works.
All internal extensions work!

The user i am having problem is connecting from his home. He has a Nat setup as well.

I have done all portforwarding on the office router.

The spa2000 tries to register with asterisk, but fails.
Asterisk debug shows, that the register request is being received and progresses on to the Authentication stage and then asterisk gives Forbidden message to the spa2000.

Xlite softphone with the same user details, registers with the asterisk perfectly from the same user.

I dont think it is nat problems, because i can see sip messages ebing handled by both asterisk and the sap2000.

It seems to be an authentication problem.

The only difference between xlite and spa2000 registration messages is the “sip uri field” in the md5 authentication.

xlite, shows the domain name
spa2000 shows the extension & ip address

Can any one help?