Back in the old days (well, about 1995), dialing this number on a BT phone, then hanging up, would cause your phones to ring a few seconds later. Picking up, you would just hear a long tone. Supposedly, this was used by engineers to test that your phones rang properly. (I used it to irritate my gene pool.) Ceased to be (IIRC) when the local exchanges were upgraded (I remember the words "from," "analogue," "to" and "digital" being thrown around, although they could mean anything). The loss of 174 was partly balanced by the introduction of 1471 - less fun but more useful.