OK, I had a little play with the router (I have to say, seems like a good router, but with no info about it on the Net anywhere!).
If you are comfortable using a command line interface, then you can SSH into your router (telnet won't work, username: user pass: user) and then try:
> voice show
Look for:
TxGain : -1 dB
RxGain : 8 dB
These are my values - check if yours are different. You might want to test different values (I don't know min/max, you'll have to experiment), but if you're not sure about what you're doing please don't do anything, this could really mess it up. I'd contact Hyperoptic and get them to change it remotely.
P.S.: Goes without saying that I'm not responsible if anything bad happens to your router. Only SSH into your router if you're familiar with CLIs.
PS2: Please don't post any screen output on here, as it will probably contain your 'landline' auth details!
PS3: Just read your post about the router being severely restricted and Hyperoptic saying it doesn't support much. I agree, the web interface changed significantly after the first overnight update (sucks!). But you can do everything via the CLI and it's not very helpful if Hyperoptic just say you can't change anything. Although any ISP first line support would say that, really....
Edited by deleted (Sat 20-Oct-12 00:51:15)