Lost of telephone

A friend of mine who has had broadband for a while, has found his telephone is not working.

The router is on a extension, and when the router is disconnected the phone works.

Now I have suggested in changing the filter, any other ideas would be welcome.

Try another router, try different sockets/filters. Try to find the common point of failure.

He should connect to the master socket with the router, then run a cable to the computer for better performance.
May not cure this problem but will improve his speed.

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Before tried anything, BT phoned them to say they had fixed a fault on their line. They had not contacted them, but saved me from a job.