I suggest that as well as the EE Bright Box Stats, you should also try to get some idea of how long your phone line is.
That information may be available along with your Distribution Cabinet ID, via-
(Some phones such as SKY are NOT included)
Regarding the "dongle", my understanding is that the xDSL/BB side is "straight-through", any Hi-Pass (Broad Band frequencies) filtering being built in to the Bright Box etc; and that it is a Lo-Pass (Low Frequency, audio only filter) on the phone side/output from the dongle.
So normally to test the Bright Box only, nothing else connected, you do not need the dongle.
However, there is a chance of failure in the Lo-Pas filter components (usually a choke/coil and capacitors) which could cause problems such as a (partial) short of the "straight-through" signal, so possibly a second test with other (new) dongles may help clarify.