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Which BT home hub do you have?
Its probably an increase in crosstalk between adjacent cable pairs which is affecting your speed, as more people are connected to the cabinet.
There is nothing anyone can do about this, as copper pairs were never designed to carry high frequencies.
Here we go...
BT Hub 6A
8 Days, 6 Hours 55 Minutes 13 Seconds
19.27 Mbps / 45.85 Mbps
19287 / 45493
6 dB / 6 dB
26538 MB Uploaded / 100304 MB Downloaded
Connection speed is a bit on the low side.
Which number matches your master phone master socket?
Single master sockets
Filtered master sockets
Is there any noise on your phone calls? Dial 17070 and select option 2, there should be no noise between the announcements.
Do you have any other phone sockets or extension wiring?
Can you please connect things up as shown on this picture, and see if the connection speed increases
Infinity - test socket connections
then report a fault to 151