BT 2700HGV VoIP - won't connect when calling it from work

[fatbellybinky]

Hi All,
I have one of the BT Business Hub 2700HGV routers that I picked up brand new at a car boot sale. With the help of this and other forums I got it all configured on my company provided broadband including VoIP so I have a phone attached to it and a local number using a Sipgate account. Everything on the VoIP side works fine EXCEPT when I try to call it from work using the X-Lite SoftPhone client on my PC using my second Sipgate account. When I try to call it, the phone at home rings but I get no indication of that on the X-Lite client at work - it just displays "trying" or "calling". When someone at home picks up the phone, there is no connection so the call setup isn't being completed. Now, I can call it from an ordinary PSTN line AND from my Laptop or desktop PC when at home using the X-Lite client and everything works fine. I even dialed out from my PC using a standard modem, having dropped the ethernet connection to the router so that I knew I was connecting from "outside" my router and that still worked. Naturally I initially assumed that it was something to do with my company's firewall blocking the inbound VoIP handshaking from my home router BUT when I swapped the router for a BT Voyager 2500V that I have, everything works fine including calling my home VoIP phone from work using exactly the same Sipgate accounts so it seems to be a router issue.
Can anybody suggest where I might start to look for a solution to this problem as I'd like to use the 2700HGV router (A badged 2-Wire router) if possible as it seems to have more features that the 2500V - provided you can find them of course as they seem to be in a lot of "hidden" web pages.

Thanks in advance for any help.