British Telecom (BT) has a line test number for BT engineers and is also accessible to customers for general line testing. This number is: 17070.
The original capabilities of the BT 17070 service were greater than they are today as BT removed several functions when the public were overusing the services thus giving problems for BT Technicians. Consequently, information like line length / distance from the phone exchange are no longer available. Note that this service is not chargeable, nor illegal to use as it is often posted on BT's own official customer forums.
The 17070 number will work on a BT line only - if you are billed by another service provider though, you still are probably on a BT sourced connection.
A message may come back: "caller ID withheld" or "There is no CLI detected for this circuit". Dial the prefix 1470 before 17070 to continue.
Recorded voice returns...
This circuit is defined as 01234 123123.
BT line test facilities.
Please press 1 for ring back, 2 for quiet line, 3 for fast test, 4 for fast cleanse or clear down.
Dial: 174 and hang up. This will trigger the Ring Back facility directly.
Note that not all exchanges and lines will have these services and some may act as a phone number check only.