Phone/broadband interaction & "noise"

[#1]

Ok, I've tried 4 different filters in the test socket (3 removed from sealed packaging) and two modem/routers ("Plusnet" TG585v7 and TP-Link TD-8816v7) with none, one or two phones connected. I have also connected the modem/routers and phones via my extension cabling and there is no appreciable difference in the RouterStats graphs for SNRM. Sample graphs (samples at 15 sec intervals) are attached.
In the first graph, the first thing of note is the blip every 30 minutes, caused by a phone going "off hook" to monitor for 1571 messages (interrupted dial tone). A similar drop of 2 to 5dB happens during calls. This, in turn, can be followed by a lower, higher or decaying SRNM level 
In the second, the graph up to 20:32 has no phones connected, then connecting a phone caused a large SNRM rise and a more stable connection. I'm thinking there's a bad joint somewhere (though the line passes Plusnet's test) which the increased phone loading partially overcame. The re-sync was caused by changing the mode from auto to ADSL2 (always uses ADSL2 and Annex A anyway) in the TD-8816.
Using a DECT phone for a quiet line test results in a short burst of decaying low level noise immediately after the recorded announcements; not noticed on a corded phone (same socket).
I would appreciate comments on this and how to "use" the fault reporting to try and resolve the problem.
Current figures are (TD-8816):
SNR Margin: 12.7 6.9 db
Line Attenuation: 61.8 37.6 db
Data Rate: 1859 632 kbps
Max Rate: 2524 660 kbps
POWER: 0.0 12.3 dbm
CRC: 1451 65452
Phil
As an aside: should it be possible to use ADSL1 (as sometimes advised)? I have tried, but the connection drops within a second or two of an apparent sync (LED on > blink period)

[#2]

If your router is in the test socket it should not be possible to connect two phones.  When you remove the faceplate to access the test socket all extensions should go dead and you can only get a working phone by plugging it into the phone side of the ADSL filter your router is using.  Can you clarify please.  Are you saying some or all of your extensions still work when you remove the faceplate?   

[#3]

I think OP is using rat's tail filters in the test socket to exclude internal fixed wiring.

[#4]

Perhaps you are correct, nitram.  Nonetheless, when one is instructed to connect to the test socket the instructions should say: "Detach the faceplate and test each extension socket by plugging in a corded phone to ensure the line is now dead".  If you detach the faceplate and find some extension sockets still work then you know you've got problems.

[#5]

Quote from: ReedRichards

If your router is in the test socket it should not be possible to connect two phones.  When you remove the faceplate to access the test socket all extensions should go dead and you can only get a working phone by plugging it into the phone side of the ADSL filter your router is using.  Can you clarify please.  Are you saying some or all of your extensions still work when you remove the faceplate?

Having re-read the post, I see the confusion:
- I do not have a cable that will allow an ADSL connection without a filter: all tests were with a filter in the test socket
- all extension cabling is to the IDC connectors on a faceplate filter and was not attached for relevant tests (with all four filters)
- use of extension cabling was mentioned to indicate there was no significant difference with/without its use
- some tests made use of a "socket doubler" to connect two phones into a filter (only two filters tested), with no significant difference
- I omitted (and not shown on the graph) that the 1571 blip can be a small increase of SRNM, though usually a drop
Phil