kev, be sure to eliminate the telephone wiring and extensions in your own home, before reporting the fault. If BT Openreach finds your internal wiring or equipment is to blame, if only in part, it could charge you £130 call-out fee.
So maybe connect your modem-router directly to the test socket of your main linebox. The test socket is revealed by unscrewing and pulling out the lower half of the face plate to the linebox. Using the test socket should disconnect all internal wiring.
Good luck at getting it fixed, kev.
The moderators of the forum have been informed of your problem. Once they have read this they may be able to help. They are a BT UK based team and if they can help they will reply via this thread asking you to contact them via a link. Once you have replied to them by the link, it can at present take up to five working days for them to re-contact you.
Hopefully, if it was Openreach who fitted a genuine 'interstitial faceplate' then maybe they installed it properly? No guarantees of course; many of BT's outsourced contractors who do this work seem to have little training nor interest in getting it right.
Though you can still remove that interstitial faceplate to reveal the test socket behind it. You would also need a BT to RJ11 lead to perform that test; I usually borrow the lead from a landline phone. That lets you 'go commando' - connecting the modem directly to the test socket - without even using a micro-filter. Sometimes those filters do go faulty, even the one in the interstitial faceplate. So you could eliminate that problem too by doing this test.
Persuading Openreach that there is actually fault beyond your home, and that you aren't just imagining it is quite hard. Just because their two minute test efforts show up no fault found (NFF) doesn't mean a lot.
Ideally you would beg, borrow or steal an EXFO or JDSU tester. That is the test equipment that competent BT engineers use to locate a high resistance joint fault, if that's what you've got. Is there a friendly engineer nearby who could bring his EXFO and perform a TDR (time domain reflectometry) test for you?
Maybe, after eliminating your own wiring and equipment as the cause, demand that the next engineer does perform a TDR test. And get him to show you the results; it's only a simple line graph with blips on it. Get him to identify the nice juicy 'blip' of the DSLAM transceiver on the graph. If he can't do that, he shouldn't be doing the test. On that TDR graph, except for the DSLAM 'blip' at the end, it should otherwise be a smooth-ish line. Every graph blip that there is indicates a potential 'issue' in the metallic path between you and the DSLAM. And your issue sounds like a high resistance (HR) joint, which should show up clearly as big unwanted 'blip' on the graph. Those HR joint faults usually occur at the Distribution Point (DP), where the D-side binder is spliced to the drop-wire that runs into your home. Joint faults here are especially common where the DP is mounted at the top of a pole, where the weather gets to it.
... or just wait until the moderators contact you and they will probably arrange for an engineer to visit and carry out all these test for you.
If I do again for the 3rd time I can't afford to be charged £130 for the call out.
First visit fault found and fixed
Second visit: Nothing found (still waiting to see if I been charged for that)
If nothing is found this time I risk of being charged twice! if not once
Third time lucky, maybe? If not, perhaps you can bill BT for all the time you've taken off work, for their engineers to attend, only to report "no fault found".
Sorry for the problems you have been having.
I can have this fully investigated for you to see if we can get a resolution. Send me an email using the contact the mods link in my profile.
Well BT have gotten in touch with me. Im not happy with what they've said.