I wish I had an answer to that.
Where there a will there is a way. He could have started it as casual conversation, he knew lots of people around London Clubs and I am sure sone of them will be unsavoury character. He could have got an idea.
Mind you I said COULD.
I don't believe it, sorry. You just need to think about it logically. He needed to be sure. That means he needed to speak to somebody with knowledge of analogue telephony engineering. He needed to conduct this conversation in such a way that the expert does not know who he is and cannot alert the police at a later point - even months down the line. It's the sort of thing the person asked would remember.
I will accept that it is possible that he could have convinced himself of the point on some level, perhaps through something as trivial as a TV documentary or engineering magazine article or whatever.
However, there is doubt here. You admit that you can't produce to me even something on a probable level that would suggest he could have obtained this information from an innocent source. A man is in prison. I totally appreciate the gravity and tragedy of the crime, but we can't just go round making assumptions like this.