Problem - only synchs when line in use.

The following setup will only synchronise consistently when the telephone is in use.

Master socket fitted with BT V10 faceplate (built-in microfilter). The line is very long and gives normal speeds of 0.25Mb but has been reasonably reliable for 2 1/2 years. Two weeks ago broadband failed but I noticed that it synchronised when someone telephoned us.
Using either the V10 faceplate or the test socket makes no obvious difference.
I have tried with 2 different routers, BT Voyager 210 and a 2-wire 393 and with 2 different telephones. Kit plugged in but phones on hook - no synchronisation.
Phone taken off hook - synchronises after 20 -30 seconds and holds whilst phone off hook. Put phone back on hook - synchronisation lost after 10 secs or so.
There is no obvious difference in synch with any combination of router and phone.

Also if no phones plugged in but just a router connected - dial in, call eventually picked up by 1571 BUT the router synchronises and holds, at least for a short while.

Any ideas as to what is wrong and where?
It may just be coincidence but the start of the problems coincided with BT working at 'our' pole when installing a new phone further down the lane (I'm told).
BT say that the splitter (whatever that is) at the exchange is fine and want to check the wiring, etc at the house with the option of charging £165+ but I can't see what of my kit could be faulty. BT fitted the V10 faceplate when they enabled broadband.

Sounds like you have a high resistance joint somewhere in the cable. Insist that BT come out to fix it, and don't accept their threats of charging.

Many thanks. I'll pursue it tomorrow morning.

If the fault is present when using the test socket (and all phones are dead at that point) then there should be no charging, unless you've damaged the external cable yourself.

The option is only (or should) be used when they resolve the fault by fixing some wiring that is actually yours to fix, it a mis-wired faceplate.

Thanks for that - it helps my confidence as I can't get MAAF to spell out exactly when/why the charge could be raised but nor could I see what else I could do to exclude anything under my control.
Today the phone has a nasty background drone (like the sound of distant propeller planes) which is a repeat of a problem of some weeks ago. I had to smile when I drove down the lane this morning as there was a BT engineer working at a pole although not on my problem which he knew nothing of. When I came back more than an hour later he was still there - sitting in his van which had broken down!

Sounds like BT spend as much on the maintenance of their vehicles as they do on their lines !