John Barefoot has a section on these in his "Telegraph Stamps of the World" book.
He lists them from 1979 to 1999, saying they were more in the way of savings stamps towards paying the telephone bill.
Used stamps do not appear to exist, but there are some with PO training bars on.

You can see more examples here: http://www.gb-precancels.org/Telegraphs/RlwyT.html#S5 These are not Telegraphic as some people believe, they are often on IR Official stamps (unlike railway telegraph cancels) and can also be found on registered mail. My best guess is that they are inspectors marks be...

Quite a few years ago there was a website doing that.
It was taken down because both SG and Scott copyright their numbering system.
There are probably various individuals with partial lists, but accuracy cannot always be guaranteed as the levels of specialization differ.