furher information:

the same problem is happening on two different Thomson phones as well as a Siemens Gigaset base station
all devices worked previously on the same voip provider (voipfone) and network (BT)

I have set the sip registration timer son the thomsons to 60 seconds to see if that helps, but as it was already 60 seconds on the gigaset along with a NAT refresh timer of 20 seconds, so I am not expecting much
(anyone know quickly the R470 times out NAT?)

the only other variable in can think of is this is double NAT's through BT's routers so I will attempt to replace one with a modem asap (although even double NAT has worked fine with the Thomson phones before)

anyone else have any voip experience (good or bad) on this box?

So your VoIP phones work but the problem is that they are not stable. I wonder that have you changed anything on your network? Do they work normally before?

We've made progress with this

It seems that a poor cable was affecting one of the phones and causing problems with a switch on our network.
It's hard to know exactly what else made the the difference as we changed a few things like turning off firewalls and ALG's and wll slowly bring them back on.

We also discovered that our voice provider allows multple devices to register per account... but would only ring one... or none... and there was no indication of why.I suspect this was the main issue.