It's not quite the same but I had problems getting usable VOIP with a Brightbox. Replacing the Brightbox router with a Draytek solved the problem instantly.
The EE network was fine, the Brightbox was crippled.
I think the Brightbox has severe limitations when it comes to using it for anything other than basic web connectivity.
Try a different router locally - I'm assuming the Draytek you refer to in your post is at the office. If not, sorry!