From aa-asterisk.org.uk wiki
17070 is a telephone number which, on BT Retail or Openreach WLR lines, is an engineering test number designed to run a number of technical tests. Over the years, the tests have varied but the basic tests are still the same.
When dialled, the message "This circuit is defined as... " is played followed a readout of the telephone number of the line that is calling, provided that the CLI was not withheld, followed by the words "BT Line Test Facility". This is followed by a menu of options. Some tests are only runnable by engineers only as a valid Openreach mobile number must be entered to run them, but some tests such as the ringback test or quiet line test can be run by anyone.
There is only very limited equipment at the exchange to do these tests so it is polite to run them out of hours and use this sparingly!