Thanks for the info. The extender is set to voice. I dissed one of the legs to turn the power off, at that point nothing was hogging the line as the extender was turned off as well. But yes, I think it must be continually trying to do something and failing.
It's the alarm that's hogging the line as if I disconnect the alarm (or plug in a phone to the extender instead) there's no issue. I say engaged as when the extender is in use, the light flashes. When I pick up a phone to listen, there is just a constant tone.
When I diss the blue and yellow below socket I get a TELECOMS FAIL message and the extender is no longer engaged.