asterisk users - Nov 2005 - Phones no longer register - except one?

Hi I've got an interesting problem.

A few days ago (maybe even a week or two) all my sip phones lost 
registrations with my asterisk box. All that is but one.

The asterisk box is out on the internet, I have two phones at my 
location and 1 at another separate location.

The only phone that remains registered is an Integrated Networks IN002 
(or something like that). This is at my location.

I also have a grandstream GXP-2000 that will not register. This is also 
at my location.

I have tried xlite and sjphone (on my desktop and mobile phone (via 
wireless)  respectivley) to test, These also fail to register.

I have a Budgetone 102 at another location which also fails to register.

There is nothing on the command line apart from the IN002 phone 
registering and talking to the * server. It dials in and out fine.

The only thing I can see that mine and the remote location have in 
common is the ISP that provides DSL (plusnet), and I was wondering if 
they were limiting traffic as they have recently announced their own 
telephony service. But I doubt it and if that was the case then why does 
the IN002 register and not the budget tone? Its a crazy paranoid theory, 
but I can't think of anything else.

The * is 1.0.9 and was working perfectly when I upgraded from a CVS version.

Any ideas - I must be missing something obvious - but I've not changed 
anything since the upgrade. Any why one phone?

Mark