Who owns the Open Reach white box?

[#1]

i.e. the one (NTE5?) which brings the phone line into my house?  I ask because the face plate has developed a fault whereby the 'hidden' master socket works when I plug a handset in, but the external one doesn't - and yet I can clearly see they are wired together and the faceplate, upon replacement, has its own male plug which mates with the 'hidden socket' inside.

Plusnet say they won't even book an Open Reach engineer (charge £50 if it's my wiring at fault) since as the master socket works then that's that - not their problem. But when I Google this I find that the little white box is Open Reach property and in fact I mustn't tamper with it - even though the Plusnet customer 'service' agent told me to take the front plate off and replug my handset into the 'hidden' socket.  Plusnet insist this little white box, clearly labelled 'BT Open Reach Mk2' has miraculously become my property, and as it's gone wrong I should just find a local electrician (?!) or someone similar to mend it.  Who is correct here? 

[#2]

Perhaps a relevant question is whether there are any wires attached to the removable lower section as, if there is, it could be your internal wiring causing the problem

[#3]

Up to and including the master socket is OR responsibility , anything after that is your responsiblity 
Kind regards

Mike

[#4]

Are you talking about removing just the lower front plate, or also removing the Mk2 faceplate, to reveal the test socket behind the Mk2 faceplate?

You could buy a new faceplate online (which would likely be the Mk3 version), if you only need the lower front part of the NTE5a, you'd probably have a buy a complete NTE5a to get one (which wouldn't be too expensive).

[#5]

Yes, the Plusnet customer service guy said to unscrew the bottom front plate to reveal the 'hidden' socket which part of the bottom front plate mates with anyway. (a) If it's BT's property I should not have been told to do this and (b) If it's their property then shouldn't they mend it?

[#6]

The bottom part of the nte5a faceplate is the separation point between the customers wiring, i.e.extensions,(which you can install yourself) and the open reach network, as "old jim" suggests if there is any faulty wiring connected to the lower part of the faceplate when you plug it back into the nte5a it will induce a fault, if you do not have any extension wiring connected to faceplate it is very rare that it goes faulty, the only time I have seen it is when the telephone cord has been forced into the socket and bent the brass strips, also damp ingress could sometimes cause a fault, so to save you a £50 call out if you could replace the bottom part of the faceplate yourself, (available on a well known auction site) 

HTH   

[#7]

You can see a guide, as to what you need to do at front of your BT Phone Book.