From aa-asterisk.org.uk wiki
NTE stands for 'Network Termination Equipment' and is officially the demarcation point at which the CPs wiring ends and your wiring begins. Openreach assign numbers to the different types of NTE that they use, other providers may use a different scheme.
Used for BT Megastream services.
Used for BT Digital Wideband services.
The NTE 5 is what most people would term a 'master socket', or BT's trade name for it, a Linebox. NTE 5 sockets come in two versions, NTE 5A and NTE 5B. The only difference is that the NTE 5B has no socket on its removable faceplate and the NTE 5A does. The two are otherwise identical.
The NTE 5 was developed to get round the provision in the Telecommunications Act 1984 that prohibited modification of BT's wiring but legally could be extended by plugging it in. It was introduced in 1986 and replaced the existing 'master socket' previously fitted. This is why the NTE 5 has a removable plate which contains a plug which fits in to the 'test socket' behind it. The test socket is officially the end of BT's ('monopoly') wiring and the removable faceplate is classed as 'liberalised' (or customer) wiring.
The standard NTE used for ISDN2 (non-Euro ISDN) services. Not available for new supply but a slightly modified version of the NTE 6 is used to provide ISDN2e service in TPON areas.
The standard NTE used for ISDN2e services, except in TPON areas, which use a modified NTE 6.
NTE 9 is the NTE that was used to provide Home/Business Highway services, which are now not available for new supply.
A new development by BT Openreach. These are currently being trialled in certain areas and consist of a grey external box on the outside of the property to which customer wiring is connected.