I have two phones (one wired, one dect) and a dect headset. All are much louder when plugged into the Hyperoptic router then when connected via BT, so fairly safe to assume it's not my hardware.
The router is an Inteno DG201 but it's severely locked down by Hyperoptic, who tell me there's no settings on it anyway. It's quite hard to find much about this router on the net too.
The main problem I have is that even on the lowest volume on my dect headset, I can't hear the beep to say it's muted, and having people talk loudly for an hour into my ear isn't much fun either!