No you cant. As mentioned by others the NTE is the demarcation point.
I think some of the confusion may be that you are allowed to change the face-plate (such as fit an adsl filtered faceplate or install an iplate), but you are not supposed to touch the backbox.. nor touch the pair that enter into the home. Extensions - you can do what you like with those since anything past the demarcation point is 'yours'.
IMHO I think that the person from BT who sent the mail is typical first line (foreign) support who perhaps does not understand the difference between the NTE master socket and the NTE5 faceplate.
If you notice during that thread some posters specifically say face plate, whilst one of the others talks about the backbox...
Bear in mind there is a certain group of users on TBB who enjoy a good stir... then also look who said that my max speed calculator simply divided by x (which it doesnt).
After a while you will get to know certain names who crop up again and again.. some of them have very interesting things to say.. some of them appear to be there for their own reasons or just to enjoy 'a good debate'
NTE stands for Network Terminating Equipment - In other works the termination point of property belonging to BT.
You are not allowed to touch the A+B wires, which you would need to do if changing the master socket. Changing the faceplate and/or adding extensions doesn't involve any work on the A+B dropwire.
BT NTE's are all specifically marked with the BT logo (these may differ over the years as BT has evolved), but all of logo'd NTEs can only be obtained from BT and fitted by a BT engineer.
Quote from BT's definitions & conditions for BT IPStream Service bt321b
... for the use of a BT provided analogue direct exchange line which terminates on a BT public switched telephone network master socket forming part of the BT Network;
“BT Equipment” means equipment (including any software) placed on a Site by BT for the provision of the Service including (without limitation) the End User NTE
“End User” means a person taking the Customer Service.
“End User NTE” means the BT Network terminating equipment on an End User’s Site, excluding any Ethernet cable which may also be provided.
The Customer is responsible for the BT Equipment and must not add to, modify or in any way interfere with it nor allow anyone else (other than someone authorised by BT) to do so. The Customer will be liable to BT for any loss of or damage to the BT Equipment, except where such loss or damage is due to fair wear and tear or is caused by BT, or anyone acting on BT’s behalf.