Crossed line on telephone

[AlwaysConfused]

I was on the phone to the bank earlier, when the operator said several times, 'can you hear that? I can hear voices, can't you hear anything?'. I reassured that I could hear no voices, other than the ones that were obviously (obviously to me, that is) in the background at the call centre. She said she was not confident in following the phone call through, as she thought there was a crossed line or an interception.

I was taken aback, but pleased that she did not want me to give out sensitive information if someone else could overhear.

I haven't come across this before. How does a line become crossed? Is there a difference between a crossed line and an interception?

[Hopkirk]

It used to be a common occurrence, but I thought new technology meant it was a thing of the past.

When I was a kid we had a shared line with our next door neighbour. If you picked up the phone and could hear a conversation, you knew they were using it.