Sorry to bang on about this, but there is a slight chance you have an incorrect installation. With a correct installation you should not need any filters anywhere. There are some recent recruits and it seems some of them are less than perfectly trained.
Did the engineer run a new cable from the master to the socket you use? If not, how long is the existing cable, as in does it go up and down walls to get from one to the other, and what quality cable is it? Did he make any changes to the wiring inside the master socket?
With a dodgy installation then phones in other sockets would be unusable in other sockets, but a filter should solve it. So there is an oddity there as well.
Have you got anything else attached to the phone line, such as a Sky box or burglar alarm?
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