off-hook broadband works

After spending some considerable time on the phone to my ISP's helpdesk
( talktalk ) the tech appeared to have improved the fault ( slow broadband
and frequent re-connects ). When I hung up the phone the broadband speed
went potty again. I finally figured this might not be a coincidence and did
several speed tests with the phone off-hook and on-hook. With the phone
on-hook the speed tests wouldn't start let alone complete. On-hook the speed
tests were fine giving me nearly 800Kb/s. ( I'm 6 or 7 Km from the exchange ).

Anyone know why taking the phone off-hook would cause an improvement?
I could sort of understand if it was the opposite i.e. if using the phone
caused the problem.

 

An on-hook phone puts a load across the line, a resistance via the hook switch, off hook
removes this load.
Ensure you have a broadband filter between this phone and the phone socket.

 

I would guess that your phone is generating noise on the line. It's even
possible that you moved a cable when using the phone, and disturbed a
noisy connection. Try using a different phone/cable/microfilter.

I've just fixed a similar problem here with the help of the Zen Internet
help-desk. I had lots of noise on the line and frequent disconnects. The
problem turned out to be a duff faceplate on the master socket, I'm now
using the test-socket instead.

Note that the subscriber is responsible for faults in the faceplate even
though it is provided by the phone company. That little tip from Zen
just saved me nearly £120. the charge for calling out a BT engineer.