Sipura3000 appears to autoanswer (not ringing)

I have several Sipura 3k's connected to PBXes. Trunks, extensions in several countries. All working great.(all but one that is)

ONE ATA extension won't ring. When I call it, it acts (on status screen) as though it's answered and after less than a minute drops the call.

Has anyone experienced anything similar?
Please note: The extension was programmed before to connect to FWD and was ringing and dialling fine. Just prior to moving to PBXes it was unable to register to FWD,

Check what settings you have on the PSTN tab under VOIP to PSTN gateway - that's the only reason I can think of which would make it auto-answer VOIP calls immediately. That, or your device may be faulty. But, I'm not an expert on SPA3000's..

Just so as not to leave this issue hanging. I've stumbled upon the reason the extensions (and lines) are not working with that site.
The router appears to have an issue with SIP (it is a Billion 743GE) Something to note for future users perhaps. I'm replacing routers in three sites (well, one to prove it then the three).