How close, physically, is the phone to the micro filter? Have you tried separating the two with an extension lead?

Are you sure that there are just 2 wires from the drop wire connected and no others are in accidental contact. Can you take a picture of the box outer and inner (be careful as it can have 70V AC and 48V DC inside). Post the images on a suitable host and provide a link.

I've not tried an extra extension to move the DECT base unit away from the actual microfilter. Don't have one. Not sure how that could help? Any logic/thought behind this suggestion that you could share with me?

Just a thought, but an A/B reversal on the internal wiring will sometimes give a 'faint' fault.....

Time to open all the boxes and provide some clear photographic evidence .......

Hello.

Yep, certain of that. The wire was (oddly) hooked up outside the house to a white telephone extension lead, stuffed and taped into a clear plastic bag and stuck behind our external cladding. This extension then fed the master socket.

Apparently BT have an arrangement with the builders so they just wire up to the house, leave the wire there, and the electricians on site do the rest.

I can't take pictures just now, but the orange and white wires are now plugged directly into the 2 and 5 connections on the master socket, which the blue and white and white and blue wires come from to feed the extension (when it was connected).

Could a dodgy master socket cause this problem? Wondering if I should perhaps try invested in an NTE5 style, seeing as we didn't have one fitted in the first place?

In reply to a post by BernieSteak:

I've not tried an extra extension to move the DECT base unit away from the actual microfilter. Don't have one. Not sure how that could help? Any logic/thought behind this suggestion that you could share with me?

The Dect phone/base station is in itself a radiation transmitter. It could be messing up the microfilter.

In reply to a post by Stevenage_Neil:

In reply to a post by BernieSteak:

I've not tried an extra extension to move the DECT base unit away from the actual microfilter. Don't have one. Not sure how that could help? Any logic/thought behind this suggestion that you could share with me?

The Dect phone/base station is in itself a radiation transmitter. It could be messing up the microfilter.

Very, very unlikely, because (a) the power output of DECT phones is very small, and (b) the frequency of DECT phones (2400MHz) is a long way away from broadband (2.2 MHz max).

In reply to a post by BernieSteak:

Apparently BT have an arrangement with the builders so they just wire up to the house, leave the wire there, and the electricians on site do the rest.

I think you've just explained the source of your problem. Builders are notorious for using the wrong wire for phones (alarm wire instead of solid twisted pair) and for getting the wiring wrong. Get it checked out by someone who knows what they are doing.

BT I assume?

From the drop wire now there are two wires, blue and white coming to terminals 2 and 5 and from them (when connected) the blue/white and white/blue wires go off to the one wired extension.

I'll have one last shot at this.

How close is the DECT phone's power brick to the filter?

Switch mode power supplies often generate broad spectrum noise and it is just possible that the ferrite cores in the filter inductors are helping with picking up some of this. I still don't know why you'd actually hear it though as that implies other problems somewhere - dodgy joints being the most likely.