Lets walk this through
You have one phone socket.
The filter is plugged in here.
The Modem & phone are plugged into this filter.
There are no other phone extensions.
If yes to all these then that's ok, just check for things like burglar alarms & Sky boxes that may have phone connections.
One thing I found with my Sister's Talk Talk connection was that they had set the connection speed higher than the phone line could cope with.
The line had been set at 8mbs when the line could only cope with 3.5mbs.
Persevere with the helpline until you speak to an engineer who can drop the line speed. When I called I told the engineer that the phone wasn't near the PC, so he phoned me back on my mobile - saved my sister 50p a minute while he did the line tests.