Filters, especially cheap ones, are notorious for failing
One way to check of course is to use the test socket behind the BT faceplate. If that resolves the difficulty then the fault is on your side and you need to fix it or pay someone to do it for you.More often than not it will be a failing filter but it can also be the cat or a mouse chewing the internal wires, or damp. Whatever, BT won't pay and will expect to charge to fix

The real answer is to fit a faceplate to the master socket that provides a separate outlet for your broadband router, isolating it from the internal phone wiring. After you've done that (for as little as a tenner) you can do away with all your microfilters because they aren't needed any more. At the same time, in many (most?) cases you'll find a good step up in broadband speeds

Down to pure chance really, if you happen to get good ones and someone doesn't smash into them with the vacuum cleaner then they could go on forever.

Some were made rubbish to begin with, I seem to recall a review site cut some open and found that one make had the ring wire connected through, meaning it could not filter properly on a regular home extension.

Personally I like the VDSL faceplate filter, it screws down, has sockets for testing, and allows for a hard-wiring a broadband "extension", but for the price you could get 6-7 cheap filters.