Microfilters causing phone quietness/fuzziness

We've got a very interesting problem and I'm stumped.

Our phones have started to suffer from being quiet and with a weird mix of fuzziness/buzz/crackles. I've been doing some troubleshooting today and have tried everything.

I seem to have narrowed it down to an issue that only occurs when the phone goes through an ADSL filter. When wired phone used directly into the master socket it's pretty clear, as soon as it goes through a filter the line goes quiet and has a lot of fuzz/crackles coming through.

Our set-up is an NTE5 ADSL Nation faceplate, with a cable going from the back of the faceplate (filtered terminals) out to separate phone extension box and cables (one for each room) all join together there. This has worked since 2005/6.

In trying to fix the fault I've tried the following:
- completely taking the extension wire from the faceplate, issue still there.
- taking off the faceplate and using 3 different filters (standard-style, two of which brand new with BT home hub a year ago and never been used) directly into the master socket, issue still there.
- Thought perhaps there might be some electrical interference affecting the filters so completely tripped the power to the house to remove possibility of that, issue still there.
- Final thought was that maybe wireless doorbell was causing it, so disconnected the batteries in that, issue still there.

Is there anything else I can try???

So quiet, when using the ADSL Nation faceplate and also when using just microfilters?

Or am I missing some odd wiring, where you have a faceplate and still use microfilters?

I'd take the phone and filters to a friends house just to double check and see if its the phone itself just not liking filters. Is this a corded phone or a cordless?

No, only quiet if don't use a filter (whether faceplate or microfilter). As if the filters are creating the issue.

Nah, the microfilters were just me trying to eliminate possible causes.

Will try filters/phone elsewhere this weekend.