I'm not sure this is the right place for this question but does anyone know whether, when you get a call waiting reply " the person you called knows you are waiting" but often doesn't respond,and then it says please try later; do you get charged as soon as you get this message or  is it only when you actually get to speak to the party you called. I have tried to find out from BT but the answers seem ambiguous.

I believe you don't get charged. I'm sure BT will fix that unless there is a regulation against it. The thing to try is to ring from a mobile. Often they will tell you how long the call lasted. If you have a PAYG phone you will know if it's taken credit.

You do not get charged to be told you are waiting, the system is set so the network you are calling from still thinks it is ringing / unanswered.  You are only charged when you get connected to the callee.

This is if it is the standard call waiting system, and not a company system that answers your call and then puts you into a queueing system.