I was asked to plug the OR modem into the test socket on the master socket
I don't want to upset you even more, as you're having a bad time of this, but in the context of your post you seem particularly irate about this bit.
There's nothing at all wrong with that suggestion. In fact it would appear to be essential in this case. The insterstitial filtered faceplate is just a modern, (easier to install), version of the normal ADSL filtered faceplate. There is nothing special about it.
All a broadband filter does is prevent anything outside the audible frequency range passing to or from the phone side of your setup. For broadband only it is perfectly permissible to have no filter.
There have been a considerable number of botched installations by the recently recruited and only half-trained Openreach engineers. I note you say an extension was installed. There could be a problem there. Do you have any other extensions as well? If so, please can we have a reasonably detailed description of what is on them, where the OR modem is on these sockets, and what else is connected?
In summary, it could be a bad installation, or it could be a faulty interstitial faceplate. It might help if we can establish which or neither before the engineer arrives. Make sure when the faceplate is removed that no other extensions work for the phone.
My broadband basic info/help site - www.robertos.me.uk
Domains,website and mail hosting - Tsohost
. Connection - Plusnet
Extra Fibre (FTTC). Sync ~ 56.0/13.9Mbps @ 600m.
"Where talent is a dwarf, self-esteem is a giant."
- Jean-Antoine Petit-Senn.
Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.