How to Test your Phone line Quality

A few times ive had problems with bt trying to do line tests and taking ages! If i get shit off them now i just explain ive already done a line test and this is how

When installing ADSL services it is always handy to be able to run a very simple test on the phone line. This test is not the most scientific, and relies on the sensitivity of the human ear, but it works great.

Learn about some simple engineer tests you can perform on your telephone line.

Before I go any further I need to point out that this test is for British Telecom (BT) analogue phone lines in the UK only. There are likely similar test services out there for other services, but I don”t know of them.

Testing from the local exchange to your building.


In order to rule out interference from inside the building it is important you test at the master socket. This socket is where the copper pair from the street enters your house. You can remove the front panel from the master socket which will disconnect all extensions in your house and reveal a single phone socket. This phone socket is a direct connection to your street.

Connect a good telephone into your master socket and dial 17070.

At the time of writing you hear a female pre-recorded voice say:

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.
[message repeats 3 times]

Select option 2 (quiet line) and listen to what you hear. You should hear perfect silence on the line – there should be no pops, clicks, whistles, buzzing and the like. If you do not hear silence first check it isn’t the phone you are using, wiggle the cable a bit to see if that affects it, if not then you know you have a problem with your line to the BT exchange. Any problems with ADSL connectivity (low speed, connections always dropping, etc) are likely caused by this.

You can now put your master socket together again and re-enable all the phones in your house.


Testing the internal wiring of your building.

So you’ve performed the test at the master socket and all sounds good.

Now you can perform the exact same test again, but this time with a phone where your ADSL modem would connect in. This should include any and all telephone extensions you intend to use.

If you hear noise on the line now you can deduce that the interference is coming from inside your house.

Things to check for are telephone cables running near transformers or motors and compressors, faulty equipment on the line introducing noise. Hopefully by removing equipment and tracing back the line you will be able to identify the source of the noise and replace/remove it.

How far from the local exchange are you?

The BT line test facility allows you to obtain your distance from the exchange. To do this dial the same line test number (17070), select option 3 (fast test) then option 1 (to say you are authorised). Then press option 2 (ring back test) and replace the handset.

After about 10 seconds you will be called back and one of the pieces of information returned will be the distance from the exchange (in kilometres). I understand BT engineers normally repeat this test 3 times and take an average. The results itself are calculated by sending a low amplitude high frequency signal down your line and listening to the farthest reflection (at your house) – therefore they are likely to vary by 100 meters or so.

The number doesn’t work for me, all I get is ”There is no CLI detected for this circuit”.

This means you have Caller ID blocked on your line. To get around this dial instead 1470 17070.