The SALT (Subscribers Automatic Line Test) is something I have not heard about for a very long time. It surprises me that it still works on your exchange.
Dialling 174 on modern electronic exchanges.will make the exchange ring back when you put the phone on hook.
On a System X/Y dialling 177 will make the exchange read out your number, and dialling 175 will produce the exchange test...
A replacement for both 174 and 175 has also been mentioned, which is
Ringing 17070 results in
"This circuit is defined as <number> " and then " BT line test facilities, please press 1 for ring-back test or clear down." Keying 1 then results in "BT ring-back test, please clear down." After a few seconds, phone rings, and announcement is "BT ring-back test completed."
When I rang 177 there was silence for about 30 seconds then it did "please hang up and try again", which was almost the same thing the first time I did 175, however it then went into reading the number etc. However when I ring 175 now there is no wait and it goes straight into line test, which would point towards some sort of initialisation first time round. I wonder whether 175 works on your line, Keith, with a wait of about 30 seconds when you ring?