Two things to consider.

Has the customer proved that the fault is with the faceplate and not on any internal extension wiring or equipment connected to the faceplate?

Is the customer on TalkTalk? They use their own contractors to do visits if they think that the fault is on the customers own equipment or wiring. This may explain a non BT faceplate fitted by an engineer.

It's the adsl V1.0

If the faceplate was fitted by an engineer acting on behalf of the line rental CP, not the broadband ISP, then it is the line rental company's problem. Not the ISP's.

Phone line and ADSL with same company. He did say which one, but I can't remember because I'd never heard of them.

He said they were insistent that it was his problem, as the faceplate was the customers property.

Has the customer proved that the fault is with the faceplate and not on any internal extension wiring or equipment connected to the faceplate?

Yes. when the face plate was removed and replaced with a standard filter, both phone and ADSL worked again.

Is the customer on TalkTalk?

No.

When did they fit it? IIRC you said recently. In which case there is legally an expectation that it works for a reasonable time, or gets replaced.

I assume he/she/you is/are happy to fit it if they send one out, so no engineer needed. But they can't fit something like that, that breaks, and just walk away.

As others have said, the easiest thing to do is replace it by one of the many reputable ones on the market, (not a real cheapo as the filter may be rubbish). I would then walk away from that provider as soon as possible. Which I would do even if they get talked into posting one out free.

In reply to a post by wingco1:

..... For the filtered face plate on a new BT NTE5 fitted by a BT engineer ? Friend of mine recently had some work done to his phone line which resulted in the fitting of a new NTE5 with the filtered faceplate.

Hang on! A new NTE5, but was it an existing filtered faceplate, not a new one? That would alter the discussion completely.

Yes, subtle diff! Ne'er the word 'new' in front of 'faceplate', only before 'NTE5'.

The engineer fitted a new NTE5, which included the filtered faceplate, sorry for the confusion. Apparently his previous NTE5 was damaged beyond repair and it cost him £100 for the replacement.

Just to add, Openreach don't raise charges against the end users, the charges Openreach raises are to the CP, as it's them that they are working on behalf of.
If the CP wishes to pass those charges in part, or fully, on to their customer, then that's between them.

AIUI there will be a 3 or 12-month Openreach warranty on the whole assembly then.